Intimacy is both the ability and the choice to be close, loving, and vulnerable. Intimacy requires identity development. You have to know yourself and your inner self in order to share your self with another. Knowing yourself makes it possible to stand for yourself in an intimate relationship without taking over the other or losing yourself to the other. This ability to be separate and together in an intimate relationship and being okay with that is called self-differentiation. Lacking the ability to differentiate one self from the other is a form of symbiosis. This too is different from intimacy though to some that kind of dependent closeness may feel the same.
From a centre of self knowledge and self differentiation intimate behaviour joins family, close friends as well as those with whom one is in love. It dwells in a reciprocity, which builds on self-disclosure and candour. However, poor development of intimacy can lead to getting too close too quickly; struggling to find the boundary and to sustain connection; being poorly skilled as a friend, rejecting self-disclosure or even rejecting friendships and those who have them.
The main forms of intimacy are emotional intimacy and physical intimacy. Intellectual intimacy, familiarity with a person's culture and interests, is common among friends. Members of religious or philosophic groups may also perceive a "spiritual intimacy" in their commonality. Some describe intimacy with the homonymous "into me see". Intimacy can also be identified as knowing someone in depth, knowing many different aspects of a person or knowing how they would respond in different situations, because of the many experiences you've shared with them.
Some lose themselves in the first flush of love. 'Falling in love' is a little different from intimacy per se. Some are engulfed by their families in a way that is not close or intimate even though it is described that way by those who are consumed by their family. The first flush of love can be like that too, but slowly the individual will assert themselves and this test the willingness of both to be intimate.
It is worth distinguishing intimate relationships from strategic relationships. Intimate behaviour occurs in the latter but it is governed by a higher order strategy, of which the other person may not be aware. For example getting close to someone in order to get something from them or give them something. That 'something' might not be offered so freely if it did not appear to be an intimate exchange and if the ultimate strategy had been visible at the outset.
Secrets are generally hostile to intimacy in a committed relationship, but not knowing of the existence of a secret, one can continue to believe there is intimacy. Maintaining the illusion of intimacy may be a strategic skill where there is an imbalance of power brought about by the existence of a secret. Knowledge is the currency of power. Betrayal of intimacy can be a traumatic experience. The person can feel cheated as well as humiliated.