Official Groups:Citizen: 
The citizen is the basic, main player in the Half-Life 2 gamemode you will choose to play as and citizens are the most commonly encountered in-game. At all times, citizens wear blue jumpsuits with City numbers (Large 45, 39, 8 which is referred to the Vault Jumpsuits in Fallout franchise) on there backs, which also looks like a orange prison jumpsuit but modified. Citizens are also malnourished due to the lack of nutrients stored in the foods (mostly in ration packets) that the Universal Union give them which decreases strength but gives them enough nutrients to live. Under the Universal Union's rule, citizens seem to be depressed at most times and are often afraid of the units. Same as the urban ways, citizens often make friends and help eachother out in order to achieve survival oppurtunities. They also live in poverty or poor conditions due to the apartments. Females with long straight hair are often cut or placed into a ponytail/bun.
Refugees: Refugees are citizens who escaped from a city. They used to be citizens until they thrived in the Outlands where they meet up with Resistance and Rebellion. They can help the Resistance or Rebellion with small taskes or offer help to them, helping them in any shape or form. As for their appearance, it looks dirty and tattered due to the encountered enemies and environment they went through, just like going through a trench.
Civil Protection or also known as "Combine Civil Authority": 
The Civil Protection are the Police force of City17 and many other cities aswell. They are humans in uniforms with brainwashed minds to do different taskes, their main objective is to clear out any citizen who doesn't follow the particular Universal Union rule within a city. Civil Protection are more strengthened from their daily and yet nutrient rations. Civil Protection have the same uniforms, as they rank higher on the ranking tree, they often get better and more useful weapons and uniforms upon their ranks. Civil Protection have a bio-signal attached to their bodies (possibly spine or head), which gives out a dispatch when deceased from accidental/purposed death or a kill from another inhabitant and is shown on a unknown radar. The Resistance smuggle the goods and especially take the vest from a dead unit, mostly from deceased C18 units. Using their stunsticks (a harsh melee weapon), they can punish citizens probably due to the failure to follow rules and is a unique melee weapon because one hit to the head can cause a lot of damage.
Overwatch Trans-human Arm: The Overwatch are called in if the Civil Protection team cannot deal with citizens who are more dangerous. Overwatch are heavily equipped with flak jackets and kevlar jackets to withstand damage from explosive splash damage and bullet hit points. Their vocoders differ from a Civil Protection's vocoder and is much more of a heavy voice, probably because of more strength in the muscles and broad body size. Armed with AR2s (pulse-rifles), they can deal much more of a threat to the Resistance and Rebellion. OTA also have 1% emotions and can very easily understand orders.
City Administrator: The administrator are picked to rule over a population of citizens, especially in a one whole city. They often speak publically using a large monitor which is shown all over the city. These loyal people are often seen to wear a brown and clean suit and have 'Doctor' (males or females). These administrators mostly live in the Nexus because they're heavily guarded by both Civil Protection and Overwatch Transhuman Arm units.
Vortigaunts: The alien Xen species, these Vortigaunts have learned to speak english and especially learn vocabulary. These vortigaunts can teleported if their Kin supports them, this is often called "Flux Shifting". Using a green lightning bolt as their primary weapon (which cannot be performed by humans), this can cause a lot of damage if it is hit upon a target. They sometimes use this respectively to resurrect a fallen human by the support of their Kin and energize eletronics such as Generators in order to reach a certain place. They have a third arm attached to their chest, which is probably used to eat humans (seen in Half-Life franchise). Using claws as their secondary weapon, this is used to open different material or to claw threats. They can even extract larvae from antlion eggs and can use bugbait which is used for humans to attract Antlion Guards, acting like a Combine thumper.
Unofficial Groups or known as "NPC"s: Classic zombie/headcrab zombie: These human creatures used to be living once like you until he was taken over by a Headcrab. These zombies often moan and scream, using their claws to slash through their opponents flesh or break open objects easily such as furniture. Zombies are often hard to take down, especially if you don't aim for the head where it's brain is located. Zombies have their chests ripped open where it can store flesh (used as food) because they can't eat from their mouths, which can be spotted in the Half-Life franchise.
More will be added soon, as for now, some of the source and such are spotted in this link.
http://half-life.wikia.com/wiki/Category:Factions
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