I found them to be exactly like the worst period pain ever, all in my lower abdomen, never had any pain in my back at all. They would come up like a wave, to start with the sensation would just be exciting, then as the wave built up it would get increasingly uncomfortable until, at the height of the wave, it would be an unbearable combination of pressure and period pain. This unbearable bit would, however, only last for a bearably short length of time, and then it was swiftly down the other side of the wave to the blessed absolute discomfort-free space in between.
Towards the end of the 1st stage and into the 2nd stage, the height of the wave started to be joined by a sensation of a cone of muscles in my lower abdomen clenching themselves - muscles working hard automatically all by themselves. This would've been the involutary pushing reflex I guess.
Unless you get unlucky and have back labour, you'll find that you are NOT in discomfort for alot more time than you are in discomfort. DD2 took 7hrs - of this, if you tot it up, I was only in any pain at all for about 1hr. The spaces in between are fabulous for regaining your spirits.
I have read that women who could describe their pain in detail reported less pain than those who just said "it just hurt!", so it's perhaps worth analysing your contractions as you have them!
One thing which I'm convinced about is that if you are frightened of the contractions, if you clench up, fight through them, grit your teeth and think "NO!", you will make the whole experience more difficult. Your body is trying to open itself up. If you clench up it will find it much more difficult to do this, and it is bound to make your labour longer, more difficult and more painful (so you'll be in more pain for longer ).
So, difficult as it sounds, your best option is always to mentally welcome the contractions, relax into them, surrender to them, tell yourself to "open!", deeply relax your body and allow the contractions to do whatever they want to do to you. This is what hypnobirthing techniques aim to help you to do.
It's also worth knowing that pain is a very subjective thing. It is well known that people can put up with a lot more if they stay mentally positive, suggest to themselves that they are not in that much pain, and think hard about other things instead. The way to make pain worse is to think negatively about it and to dwell on how bad it feels.
I remember having terrible pain with a broken wrist, but I sat and managed to become engrossed in a book. I finished the book and came out of the "book dream" and all of a sudden the pain came flooding back. I remember distinctly the sudden change painfree to pain purely from having been thinking about something else. Can't read a book in labour of course, but this "getting your mind away from it affect" is the other thing that hypnbirthing techniques try to achieve.