Under normal conditions, your battery is designed to vent excess gasses to prevent pressure from building up on the plates. Unfortunately, this process allows harmful acids and gasses to reach the metals on top of the battery case, more specifically the battery terminals and fusible link. This causes a build up of corrosion which in short time can erode the metal terminal and reduce the terminals contact patch to the battery post causing no-start or hard start conditions.
Because the negative terminal is crimped to the battery ground cable, replacement of the entire cable is required to replace the terminal.
Images to the right are generic and may not represent the exact part you recieve.