The Division Enemy Types and Factions

There are three basic types of NPC enemy in the game:

Standard enemies - Essentially cannon fodder, they have a single health bar and are easily dealt with.
Veterans - These guys have a secondary armour bar which needs to be depleted before you can actually cause them damage, so armour piercing mods are useful here.
Elites and Named Enemies - Like Veterans, Elites are equipped with armour, but they're generally tougher and smarter into the bargain and often have a special attack or two up their sleeves. Fortunately, they also tend to have a weak spot which can be exploited.

Each of these enemy types may have a class assigned to them which will be marked with an icon next to their level indicator. Lightning means they're going to rush at you and try a melee attack, a targeting reticule means a sniper, a grenade means they'll be throwing things at you, a wrench means they'll deploy turrets, and enemies sporting a cross are medics. These should be taken out first, as they'll heal themselves and their comrades, turning what might otherwise be a simple fight into a protracted struggle.

If you stray into Districts that are beyond your level range you'll likely run into enemies which have a skull icon next to their health bar. That icon is there for good reason, and is a sign that either you should run away or that you're about to die...

Enemy Factions
Rioters are the default enemy type, and they're armed with fairly standard weapons. They are also less likely to be wearing armour, and generally don't do anything unexpected in battle.

The Rikers are escaped convicts, so they're a little tougher built than the rioters, but still use fairly basic weapons, They're more likely to try and flank than rush straight at you.

Cleaners are middle-tier enemies and carry flame throwers in addition to the normal arsenal. They tend to move more slowly than the other types, but flamethrowers can pass over or around cover at close range and will apply a Burning effect - avoid getting up close and personal if you can.

Lats Man Batallion [LMB] - these guys wear armour and good quality Gear, carry powerful weapons, are more likely to have medics and snipers and their squad, and are generally a lot tougher and smarter than everyone else. Approach with caution.