I compare them to super intense period cramps or the crampy feeling you get when you have really bad diarrhea. Sorry thats a gross analogy but it's true for me anyways.
They can be different depending on the woman, every labor is so different. For my son I had no real warning, I got a few strong cramps then all of a sudden I was having contractions. I knew they were contractions, there was no doubt in my mind, but mine were different then most from what I understand. My contractions started at 3-4 minutes apart and about 40 seconds long, I had absolutely no pre labor. Like I said there was just no mistaking them, it was an intense period cramp feeling, and they were easy to time. Some woman experience prelabor and slow labor in the begining, which can be harder I think to be sure your in labor. Contractions can be spuratic, and spread over days before they tighten up and become consistant.