Marx's father, a lawyer, converted from Judaism to Lutheranism in 1824. Marx studied law at Bonn and Berlin, but became interested in philosophy and took a Ph.D. degree at Jena (1841). He early rejected the idealism of Georg Wilhelm Friedrich Hegel and turned toward materialism, partly through the influence of Ludwig Feuerbach and Moses Hess.
In 1842 he became editor of the Rheinische Zeitung, but his demands for radical reforms led to its suppression in 1843. He then went to Paris, where he began his lifelong association with Friedrich Engels. At this time Marx became a socialist. He devoured the works of Adam Smith, David Ricardo, the comte de Saint-Simon, and many others. Antagonized by the individualistic radicalism of Pierre Joseph Proudhon, Marx attacked him in The Poverty of Philosophy (1847, tr. 1910), an early attempt to systematize his own thought. In this period also he wrote, with Engels, The German Ideology (tr. 1933), which provided an exposition of his dialectical materialism. Breaking with the tradition of justifying social reform by appeal to natural rights, he invoked "inevitable" laws of history to predict the eventual triumph of the working class.
In 1847 Marx joined the Communist League and with Engels wrote for it the famous Communist Manifesto (1848), which strikingly expressed his general view of the class struggle. The failure of the revolutions of 1848 convinced Marx of the need to stimulate the consciousness and solidarity of the working class through the founding of open revolutionary parties. Exiled from most continental centers, he settled permanently in London in 1849. He lived in poverty, made more bitter by his own chronic illness and the death of several of his children. At times he was able to earn funds as a correspondent for the New York Tribune, but he was continually dependent on Engels for financial aid. Nonetheless, he pursued research in the British Museum and continued to write steadily.
In 1864 Marx helped to found the International Workingmen's Association. Through this First International and through the work of Ferdinand Lassalle and others, Marx's ideas began to gain primacy in European socialist and radical thought. This primacy was greatly furthered with the publication of the first volume of Das Kapital (Vol. I, 1867, tr. 1886; Vol. II-III, ed. by Engels, 1885-94; tr. 1907-9). The manuscript for the fourth volume was edited by Karl Kautsky and published as Theorien über den Mehrwert (3 parts, 1905-10; tr. of 1st part, A History of Economic Theories, 1952). A monumental work, Das Kapital provided a thorough exposition of Marxism and became the foundation of international socialism.
As Marx's reputation spread, so too did public fear of him. He insisted on authoritarian sway within the International, and finally, after controversy with Mikhail Bakunin, virtually destroyed the International for fear of losing control over its direction. He remained the prophet of socialism and was often consulted by the various socialist party leaders. His role was frequently that of urging more hard-minded policies, further removed from bourgeois embellishments; The Gotha Program (1891, tr. 1922), a critique, illustrates this position. The complexity and vituperation of this polemic characterize much of Marx's prose. In his last years Marx's great intellectual vigor continued unabated. The importance of his dialectical method and of his theories goes far beyond their immense political influence; many scholars consider him a great economic theoretician and the founder of economic history and sociology.
There are many translations and editions of Marx's best-known works and of his and Engels's selected correspondence. See the Collected Works of Marx and Engels (40 vol., 1975-83). The standard biography of Marx is that by F. Mehring (tr. 1935); other notable works include those by O. Rühle (tr. 1929), E. H. Carr (1938), C. J. S. Sprigge (1938), K. Korsch (1939), and I. Berlin (4d ed. 1978). Recent biographies include those by R. Payne (1968), D. McLellan (1973), P. Singer (1980), A. Wood (1985), and F. Wheen (2000). See also bibliography under Marxism.
Marx's theory relies on Feuerbach's The Essence of Christianity (1841), which argues that the idea of God has alienated the characteristics of the human being. Stirner would take the analysis further in The Ego and Its Own (1844), declaring that even 'humanity' is an alienating ideal for the individual, to which Marx and Engels responded in The German Ideology (1845).
Marx's Theory of Alienation is based upon his observation that in emerging industrial production under capitalism, workers inevitably lose control of their lives and selves, in not having any control of their work. Workers never become autonomous, self-realized human beings in any significant sense, except the way the bourgeois want the worker to be realized. Alienation in capitalist societies occurs because in work each contributes to the common wealth, but can only express this fundamentally social aspect of individuality through a production system that is not publicly social, but privately owned, for which each individual functions as an instrument, not as a social being:
'Let us suppose that we had carried out production as human beings. Each of us would have in two ways affirmed himself and the other person. 1) In my production I would have objectified my individuality, its specific character, and therefore enjoyed not only an individual manifestation of my life during the activity, but also when looking at the object I would have the individual pleasure of knowing my personality to be objective, visible to the senses and hence a power beyond all doubt. 2) In your enjoyment or use of my product I would have the direct enjoyment both of being conscious of having satisfied a human need by my work, that is, of having objectified man’s essential nature, and of having thus created an object corresponding to the need of another man’s essential nature. ... Our products would be so many mirrors in which we saw reflected our essential nature.'" (Comment on James Mill)
In this work, written in 1844, Marx sought to show how alienation arises from private labour, from commodity production. He argued that "My work would be a free manifestation of life, hence an enjoyment of life. Presupposing private property, my work is an alienation of life, for I work in order to live, in order to obtain for myself the means of life. My work is not my life." (Comment on James Mill)
Marx attributes four types of alienation in labour under capitalism. These include the alienation of the worker from his or her ‘species essence’ as a human being rather than a machine; between workers, since capitalism reduces labour to a commodity to be traded on the market, rather than a social relationship; of the worker from the product, since this is appropriated by the capitalist class, and so escapes the worker's control; and from the act of production itself, such that work comes to be a meaningless activity, offering little or no intrinsic satisfactions.
Alienation is a foundational claim in Marxist theory. Hegel described a succession of historic stages in the human Geist (Spirit), by which that Spirit progresses towards perfect self-understanding, and away from ignorance. In Marx's reaction to Hegel, these two, idealist poles are replaced with materialist categories: spiritual ignorance becomes alienation, and the transcendent end of history becomes man's realisation of his species-being; triumph over alienation and establishment of an objectively better society.
This teleological reading of Marx, particularly supported by Alexandre Kojève before World War II, is criticized by Louis Althusser in his writings about "random materialism" (matérialisme aléatoire). Althusser claimed that said reading made the proletariat the subject of history (i.e. Georg Lukacs's History and Class Consciousness  published at the Hungarian Soviet Republic's fall), was tainted with Hegelian idealism, the "philosophy of the subject" that had been in force for five centuries, which was criticized as the "bourgeois ideology of philosophy".
The propertied class and the class of the proletariat present the same human self-estrangement. But the former class feels at ease and strengthened in this self-estrangement, it recognizes estrangement as its own power and has in it the semblance of a human existence. The class of the proletariat feels annihilated in estrangement; it sees in it its own powerlessness and the reality of an inhuman existence. It is, to use an expression of Hegel, in its abasement the indignation at that abasement, an indignation to which it is necessarily driven by the contradiction between its human nature and its condition of life, which is the outright, resolute and comprehensive negation of that nature. Within this antithesis the private property-owner is therefore the conservative side, the proletarian the destructive side. From the former arises the action of preserving the antithesis, from the latter the action of annihilating it.
Alienation is a theme in Marx's writing that runs right throughout his work, from the Economic and Philosophical Manuscripts of 1844, to Capital - especially the unpublished sections entitled Results of the Immediate Process of Production. An online archive of almost everything written by Marx can be found at the Marxists Internet Archive- at which you can search for 'alienation'. Another good way to approach Marx's original writing is through a good collection - Karl Marx: selected writings (second edition), edited by David Mclellan clearly indicates sections on alienation in its contents. Key works on alienation include the Comment on James Mill and The German Ideology. An example of characterisation of alienation in Marx's later work (which differs strongly in emphasis, if not in actual content from earlier presentations) can be found in the Grundrisse. Marx's work can sometimes be daunting - many people would recommend reading a short introduction (such as one of those indicated below) to the concept first.