Gestalt therapy is based on a very complex philosophy developed by Fritz Perls, his wife Laura, and others in the 1940s-50s. (It found new life in the 70s when Fritz Perls was deeply involved in the Esalen Institute in California.) Rather than attempt to explain the philosophy (done concisely on Wikipedia) it may be more useful to present a few examples of how Gestalt therapy is used in practice.
Gestalt's homage to Eastern philosophy is its emphasis on the present. Reality is what is happening in the here and now, not in the past or future, and it is always shaped by the individual's own perceptions of him/herself and of the "other." The word "Gestalt" means the whole," or the sum of an individual's hidden and not so hidden feelings about an issue affecting him or her. (There are no "did your mother breast feed you until third grade" questions in Gestalt therapy!)
The "empty chair" exercise is a commonly used Gestalt technique. The therapist asks the client to address a conversation to an empty chair which represents a person, a goal, a desired outcome, etc. The client moves back and forth between both chairs, playing himself in the moment and the "other" or issue. The point of the "conversation" is to reveal and explore the emotions that are attached to the "issue" and to find ways of taking personal responsibility for resolving conflict. The concept that we are responsible for our own feelings is a core Gestalt belief. So, a client might explore issues involving a dead parent by playing both roles in a conversation, and observing and working with what feelings arise during the conversation. It doesn't matter if the "imaginary" person would have ever said the things the client is saying, what matters is that the client's saying them represent the feelings that are present NOW. Therapist and client may use this exercise over and over until the client feels that s/he has resolved the conflict, and can "own" the outcome, rather than continuing to place blame on the "other."
Fritz Perls also encouraged the use of dreams for Gestalt work to reveal "undiscovered" parts of an individual's personality. Similar to the empty chair exercise, a client uses a dream or fragment of a dream and acts out all the parts, the more vividly, the better. In a way, both techniques are ways of having a conversation with one's self and one's perceptions, until an important piece of information or solution is uncovered.
This is a very simplistic "thumbnail" of Gestalt. But if you remember that reality is what you think and feel NOW about yourself and the "NOT yourself" or "other" and that you are responsible for your emotions-then you understand the essence of Gestalt.
Again, thanks to Wikipedia and several other sites, the following being really good: http://westhartfordcounselingcenter.com/gestalt.html
for giving me a brush-up on Gesalt. It's been a long time since graduate school!