Mine started with my back having a dull pain for a while (I hardly noticed) then I started getting cramps in my lower abdomen and after a little while I noticed that they were coming and going and getting worse.
Contractions are differen't each pregnancy, each woman, and each child. You can have subtle pains or horrible pains. It can be back pain or shoot across your belly. It's hard to really describe. My first set I thought I was cramping and my mom knew I was wrong and laughed at me for it later. It really depends honestly.
With my first was all back labor and that was okay for the most part. With this one I had contractions for 2 weeks before I had him and yes they are painful my whole stimach felt like a huge knot that was tied real tight.