I understand there are a number of protein-protein interactions, but what types of interactions exist? and what are the characteristics of them?
There are so many types of protein-protein interactions via various domains, such as SH2 binding (in RTK signaling), Pleckstrin Homology domain (involved in signaling) among others. This site gives a nice list: http://pawsonlab.mshri.on.ca/index.php?option=com_content&task=view&id=30&Itemid=63
Of course, protein-protein interactions rely on the premises of basic biochemistry:
There are many ways to study protein-protein interactions, including, but not limited to