The following is a very brief overview of combat for Pathfinder players.
A "turn" in Hero System is 12 seconds long. This is roughly analogous to a round in Pathfinder (which is 6 seconds long).
Each turn in Hero System is divided into 12 "segments", which are each one second long. A "phase" in Hero is a segment in which a character gets to act.
Movement and attacking work similarly to Pathfinder. In any given phase, a character may move once, then attack, or move twice.
Hero System doesn't have a built-in mechanic for attacks of opportunity, although it would be possible to implement them with a triggered power.
You may hold an action either to trigger at a condition (equivalent to a readied action in Pathfinder), or wait until a time of your choice, like delaying an action in Pathfinder. You may never act more than once in a single phase.
You may abort your next action in order to take a half-phase defensive action, such as using a defensive power (throwing up a barrier, raising defenses, etc), blocking, or diving for cover.