Snort: How do I log certain types of traffic?

["Angel Gabriel" <badmangabriel lycos co uk>]

I would like to log certain types of traffic that runs across my network,
and the later analyze it. *The purpose of me doing this? None, except
experience!*

I would like to know how I would go about this, and can snort do this? For
example, I'd like to log all Microsoft Mesanger packets, and read them back
later, or maybe log every packet to a certain machine, or maybe even the
interaction of my clients with the servers. Can this be done?