Hi K. - I'm 33 weeks and have been having BHs since week 29. This is my second pregnancy; during my first pregnancy I had them too but I thought it was my baby's big ole butt shoving against some part of my body *laugh*. Little did I know that they were BH. The BHs are normal - it's like "practice" for your uterus before the big day.
My BHs feel like they are a hard ball, slightly swelling and tight. They are uncomfortable and sometimes painful but not painful like regular contractions can be (just like the previous poster, I too would get annoyed when people would say "you'll know when you're having a real contraction!", but then once I experienced real contractions, um yeah, the difference was pretty clear!). I find sometimes that my BHs take my breath away and it feels like sometimes my bladder is being pressed.
The difference between BHs and regular contractions is the pattern and regularity. Usually the BHs don't last very long and when it is over, wait about 5-10 minutes or so to see if you get the sensation again. You may have a few BHs within an hour, but there isn't much of a 'pattern' to them (meaning they don't last about the same amount of time, the downtime in between is not regular, etc). Also, if you move or change positions they tend to go away.
Good luck and enjoy the last 6 weeks!