Labour begins when the uterus starts contracting regularly. These contractions then get stronger and more frequent.
Contractions allow your uterus to push the baby downwards and to pull the cervix upwards, allowing the exit to get bigger and bigger in preparation for your baby's head.
Most labours start off quite slowly and gently, and you may have several hours of wondering if this is actually 'it' or not.
Some women experience quite powerful Braxton Hicks 'practice' contractions. It's not uncommon to think this is real labour; some women even end up in hospital, only to be sent home because nothing's happening. Don't be embarrassed if this happens to you - maternity staff have seen it all before.
When the real thing does start, if you've had a normal pregnancy so far there's no need to make a mad dash for the hospital. You'll probably cope better with the contractions if you're free to walk around and choose your own distractions.