Help a novice: How do grading systems work?

I am not a super experienced climber either, but I hope this helps, Katie can probably answer a bit better but...

For Bouldering - it is on a V scale. V0 being the easiest, and I am pretty sure it goes up to about V15 being the very very hardest. I think the jump to the next number gets harder and harder as you go up.

For Sport Climbing - it is on a 5. scale. I am not entirely sure where it starts, I think maybe 5.5 and goes up to 5.15 too. There are a,b,c,d in between making a 5.11d harder than a 5.11a.

I hope that helps! I can't really explain trad becuase I don't do it. :)

Thanks Whitney. I also found a couple pages on Wikipedia that explain a lot

http://en.wikipedia.org/wiki/Grade_(climbing)

http://en.wikipedia.org/wiki/Grade_(bouldering)

Whitney pretty much covered it. The only thing I would add is that the 5. scale comes from climbing being considered 5th class -- meaning that you need a rope for safety. A lot of hikes are rated from Class 1 to Class 4 (class 4 being like scrambling, but nothing technical where you would need to rope up).
As far as traditional climbing goes, it's rated the same way, until you get into aid climbing, which I try to avoid and therefore don't know too much about.

I don't know if anyone even reads these older threads, but for what it is worth I thought I would add a note here. I have always found class 4 climbing in the Cascade and Olympic mountains here to be indistinguishable from low class 5 climbing. Fred Becky (a well known local climber/author) says, "Class 4 involves intermediate climbing where most climbers will want a rope because of exposure and sometimes use pitons at belay points. Some Class 4 climbs would be certain minimum Class 5 if it were not for the security of ledges or the availability of ....protection." Of course the comment about pitons is a bid dated, but you get the idea. JIM