After your visit to Kashyyyk in the story bounty hunters will start to spawn around the various planets in the game. They usually spawn in groups of two.
There are only three types of bounty hunter:
Bounty Hunter - has a jetpack, shoots missiles and has a blaster
Commando - Has flashbangs, a sheild and repulsor blasts, uses a blaster
Bounty Droid - Uses charge attacks and hits heavily.
They can be tracked in the Tactical guide provided you scan them after defeating them.