In Lasik they create a flap by cutting the cornea and then they use a laser to reshape it, then they reposition the flap back in place.This way the flap acts as a bandage on the wound and reconnects with the cornea. The Flap is never actually healed and may be dislodged even years after the surgery - Although will need significant force like a direct poke in the eye, severe car accident etc.
In PRK a flap isn't created, the laser reshapes the cornea directly (After removing a small epithelial layer) and then a special bandage contact lens is applied to the wound. This is generally considered safer (Nothing to be dislodged), But the healing process is much slower and the time it takes until your eyes are at their best capacity is much longer. As i previously wrote, I was back at work the next day and my sight was almost perfect after 3 days. My cousin had PRK, the total healing time was around 2-3 weeks, and in the first week she could barely open her eyes.
PRK is also considered a little less accurate, but i am not sure why... For a nice comparison see: http://www.lasereyefacts.com/prk-vs-lasik.html