The game uses the combat dial system developed by Wizkids. Mage Knight also uses this system. The figures are on stands with dials, and the dials are turned if a piece is successfully attacked. When the dial is turned the stats have changed.
There are cards with the modern pieces. They elaborate on stats.
Heroclix World is a popular site for fans. It features news about the game, articles, interviews, rules, mods, and more.
Modders enjoy changing the miniatures. They might attempt to enhance a sculpt or even turn it into something new. They could change it into another character or even something of their own creation. They often need to make a new dial when they mod, changing different stats to be more relevant to the new piece.
There was a game called Horrorclix in the same vain as Heroclix, but it was discontinued in 2008.
Heroclix does limited edition and event only figures or sets sometimes. They even have a Stan Lee and a Joss Whedon!
Heroclix aren't all small. They have colossal figures, like this one.