Braxton-Hicks contractions feel like a tightening of your abdominal muscles. They can be uncomfortable, but they do pass after a fairly short time.
Labor feels like you're passing a basketball. It's uncomfortable, and it ends when your baby is born.
When you go into active labor, you'll feel the difference between false labor (Braxton-Hicks contractions) and real labor. It's hard to describe, and Braxton-Hicks and active labor each can feel different to different women. If you have any doubt about whether you're in labor, call your obstetrician!!!!!! She or he would far rather have you come into the office 10 times on a false alarm than decide that this is just a Braxton-Hicks and wait until you're in a state of emergency with the real thing.
Good luck to you and your baby!