Acquaintances at Work: The more we are in the working life, the more HI-BYE friends we make. The kinds that you probably meet during meetings, friends only DURING the meeting, people you meet at parties and outings. People you probably couldn't care less for if anything happened to them. And yet, they could be people you need to get your work done, therefore you keep their namecards for sure.
Your other half's friends: Your bf's/gf's friends become your friends automatically. You either try to get along or you click with them anyway. But just how civil or how nice or how friendly you wanna get at the end of the day, also depends on how u click with that person, no such thing as courtesy call. You can be fake once, but you can't be fake forever (if that's how the relationship with ur bf/gf is gonna last). There's no fast rule that says you have to buddy up with them. But if you're compatible with ur other half, the chances are you'll like his/her friends anyway.
Friends With Benefit: OK, not just applicable for people who are there for convenience and non-commital sex, but also friends who are your friends because of business partnership. Friends whom you may not necessarily like but is influential enough to help you to GET THINGS DONE. Just how friendly you wanna get with these kinda people is really up to the individual.
GOT ANY OTHER TYPES I SHOULD KNOW?
Virtual/Blogging Frens: Almost forgot about this, thanks to Jemima. How many of you have friends on the blogosphere/msn/irc that you've never met before and yet connect spiritually (at least in writing?) sometimes it is these people who gives you the most emotional support. I dunno about you but sometimes typing it out is easier than saying it out. For those who are verbally challenged (quiet) maybe this is an avenue they can go to to find support and love. Well, why not. It is the 21st century.