Since I feel more like typing instead of cleaning, I'll answer your question.
The 5th string is a B because it keeps the bass tuned in 4ths, AKA 5 frets apart. This means all of your fingering patterns will still work no matter where you start on the bass, so long as you still have enough strings and frets to complete the pattern.
Some people think it should be a D, like an "alternate E string with a different tuning", and there might be a few people who tune it like that, but it's a big handicap to have that E string (that you DON'T want to accidentally hit now) in the way.
Some people think that, as a B string, it should be a HIGHER string, like a guitar, and tuned to B above the G string. There are a few players who do this as well, but they're either converted guitar players who miss that weird little intervallic relationship between the G and the B, or they're bassists who play chords behind their band and PREFER the relationship, as it gives them lots of chord shapes to play with (Tim Bogert is one of these who does it with great success).
No, the lion's share of 5 string basses are strung with a LOW B. The Low B is tuned to about 31hz and is the same as the 2nd white key on a piano from the left.
The extra notes a B string gives you aren't always necessary for a given song - they're pretty stinkin' low! - so oftentimes you'll see players ignore that string for a few songs. But it's a very nice "room filling" effect when you want it.
A lot of people buy 5 strings because they think they're "better" - but my contention is, if you're not hearing notes down there, and you're not re-tuning your E string all the time wishing you could hit those notes, you probably don't need a 5 string. Lots of bassists play their entire careers without owning a 5er. It's not a big deal.