Both precipitation and agglutination are immunoassay techniques that involve using the antigen - antibody complex to identify and separate proteins.
For immunoprecipitation, one must first label the sample by adding a marker element or radioisotope. The sample is then incubated with the antigen, and antigen - antibody complexes are allowed to form. The complexes are then passed across a separation plate which contains a bound protein which has a high affinity for the antibody - antigen complexes. The complexes are separated from the bound protein by centrifuging, heating to disrupt the protein, or pH adjustment, and are collected.
Agglutination is a simpler version of the same technique. To do it you mix an agglutinin with a liquid blood sample, and if the corresponding protein is present in the sample, you will see clumping. It is useful as a rapid diagnostic test for things such as blood group typing.
If you need more details on the techniques, you will find an excellent reference from Tulane University here.