They are generally a painless tightening of the uterus. Usually, they happen up high, in the fundus and spread down, but that can vary from uterus to uterus. Essentially, they are your body's way of getting itself in shape--since it's an involuntary muscle, you can't exercise it. It has to do that itself. For some women, they are frequent, and for some they don't start happening until just before labor begins. For some women, they get BH from the time the are pregnant, but you cannot feel them until your fundus is high enough to be outside your pelvic cavity. Hydration can help them not feel painful, as can resting or changing position.
For me, they are totally painless (at least compared to real contrax.) and they take my breath from pressing on my diaphragm. Real ctx take your breath for another, ahem, reason. :LOL
They are similar to the contractions that you have after orgasm, so you can compare them to that.
HOpe this helps!