Bandits are rural raiders who live around Jackson County and attack Tommy's settlement and its hydroelectric dam. You fight them in Tommy's Dam, first at the dam and then at the ranch house.
Expect armored targets and smoke. Armor piercing rounds and the shotgun help against the armored ones, and a smoke cloud from them is your cue to move rather than hold a position. The dam's workbench is destroyed in the attack, so upgrade before the raid if you can.
Maria warns Joel that raids are common in the area. When the bandits hit the dam, Joel, Tommy and the workers fight them off, though several of Tommy's people die.
FEDRA
FEDRA, the Federal Disaster Response Agency, is the military government that runs the Boston quarantine zone. Its soldiers are the best-equipped enemies in the first chapters, and you fight them while smuggling Ellie out of Boston.
Hunters
Hunters are the armed civilians who took Pittsburgh from FEDRA and now ambush anyone passing through to strip them of supplies. You fight them through the Pittsburgh and The Suburbs chapters.
Infected
The Infected are people taken over by a mutant Cordyceps fungus, and they are the main threat between every human fight in The Last of Us Part I. In Part I and Left Behind you meet four stages: runners, stalkers, clickers and bloaters, each harder to kill than the last.
Runner
Runners are stage one infected, people turned within the last couple of days. They still see, move fast and swarm in packs, so the danger is numbers rather than any single runner.