What is a 'species'?

It's Friday, I've worked hard this week and got lots done, so time for a bit of fluff. Although this is a serious real-world question, it seems to come up most often (and get most badly misunderstood) in a roleplaying context.

For instance, I've seen pedants chide roleplayers for talking about elves (Tolkien/D&D/generic roleplaying variety) as a separate 'species' from humans. After all, they can interbreed, right?

( What is a 'species'? )

Nearly there...

My birthday always sneaks up on me. It's early enough in the year that by the time I've recovered from Christmas and new-year stuff, it's almost upon me and I don't get around to planning anything much for it.

But this year, sandypawozbun has been considerate enough to have her wedding the weekend after, so I get to go to a party anyway. Yay!