Repeatedly cracking knuckles can cause inflammatory damage and swelling to the ligaments, giving the appearance of slightly thicker, fatter fingers. Other effects include loosening of grip and the stretching or injury to the ligaments.
Knuckle cracking is only harmful if it is done excessively. When the knuckles are cracked, synovial fluid rushes to the gaps between the stretched tendons and ligaments in the fingers. To make room, gas bubbles found in these gaps burst, causing the popping sound. Over a period of time, it can become harder for the joints to stretch back after the continual pressure applied to crack them.