For many HeroClix figures, there are symbols on the back of their dials that represent the various team of characters they are associated with. These are collectively known as team symbols, and they impart yet more abilities to your HeroClix characters. Here are some common team symbols you might see:
From left to right: Batman-Ally (grants Stealth); Avengers (allows a character to move without taking up an action for the turn); Superman-Ally (ignores Hindering Terrain); X-Men (can take an action to heal each other, with a dice roll to determine if the "healer" gets hurt)
Team Symbols/Team Ability References
For more on team symbols, including a fully-updated list of team symbols and abilities currently in the game, check HeroClix.com's Rules Downloads section--look under section 5 of the Player's Guide, labeled "Team Abilities," for a handy PDF.