A karyotype shows the shape, relative sizes and general structures of an organism's chromosomes. This technique, which uses special dyes and microscopes to view the chromosomes, is very useful for detecting large mutations or other abnormalities in an organism's genes. The chromosomes used are taken during a phase of cell division when they are the most condensed, as otherwise portions could be unwound for use in creating protein molecules.
A karyotype is very different from a genetic analysis, despite the fact that the two processes are highly related to one another. Genetic analysis examines the sequence of the genetic code in particular genes. The chromosomes examined during a karyotype contain huge numbers of genes, and nothing specific about the genetic code can be learned from the examination. Nonetheless, the dyeing process creates banding in chromosomes which, together with their shape, do indicate any large abnormalities. Any particular species has a typical appearance for their chromosomes, and large deviations from that indicate a problem.
One condition that can be diagnosed via karyotype is aneuploidy, which is a condition where an organism has the wrong number of chromosomes. Down's syndrome is probably the most well-known example of aneuploidy. This is the simplest condition to detect via karyotype, but many other conditions can be indicated as well.