Immunoprecipitation is am experimental technique that isolates a particular protein from a homogenate of thousands (or more) by using a specific antibody against that protein, causing it to precipitate out of solution. Going from this, chromatin immunoprecipitation (ChIP) is a particular type of immunoprecipitation that aims to identify protein-DNA interactions. This is accomplished by covalently bonding the protein with DNA , followed by shearing of the DNA into segments and isolation of the protein-DNA complex via specific antibodies against the protein.
Most often the DNA-binding protein and DNA itself are cross-linked by either formaldehyde (causes linking between primary amino groups) or UV-light. For shearing the DNA into manageable pieces, most experiments use sonication. However one could also use nuclease digestion, which is useful for mapping DNA-histone interactions.
Once the protein-DNA complex has been isolated, the DNA segment can be identified and sequenced.