Little Snitch for the Mac is both great and terrible. It's great because I know exactly what apps are doing with my network. It's terrible because there are a lot of legitimate processes making calls home that freak me out. For example, the "apsd" process started showing traffic in Little Snitch. I decided to temporarily allow it while I investigated further.
I could just block everything, but setting blanket rules in Little Snitch can cause frustratingly complex problems. I'd rather make intelligent decisions. I use two very good resources for finding a Mac service by name and getting more info about what it does.
Both of these sites seem a little old and perhaps not well maintained. I also can't vouch for anything they are trying to sell. But, they do have a huge browsable list of Mac related system processes. If I find a reference, I then add a note to my Little Snitch rule.
Another great reference source is the built in man pages available through the Mac terminal. Just punch in
man apsd to get a pretty reasonable description of the process.