Mobs are, generally speaking, moving entities. In SS13 they can be separated in various categories of complexity, each of which has it subcategories. The categories are named as they are in the game code.
Non-Interactive Mobs
The most basic form of mob, it's generally unable to interact with other elements of the game, unless they have specific skills that allow them to do so.
Ghosts
Ghosts, or observers, are what a player controls while dead or observing. They have unlimited movement, cannot be damaged or destroyed, and are unable to influence the round in any way (with rare exceptions, such as wizard events, ouija boards, and so on).
Communication
Ghosts can hear and understand everything, but can only be heard by other ghosts or by using arcane means of contacting the dead.
Living Mobs
Living mobs are the simplest form of "actual" mob.
Health
Living mobs usually have a single pool of health; any kind of damage taken will impact that pool, and when it runs out the Living Mob dies. While playing a Living mob the health amount can usually be seen in the Status tab on the right as a percent value. Losing health has no negative effect by itself; it only matters if it runs out. Their health can be seen by using a Diagnostic HUD.
Status Effects
Living mobs are generally immune to all status effects, (with few exceptions, like drones).
Interaction
Living mobs are locked into a single intent. They can usually only do a single action (generally attacking) through clicking. A few mobs have a ranged attack, that is activated by clicking at a non-adjacent tile from the mob.
Communication
Living mobs are generally able to speak normally, and can use devices like intercoms with the .i prefix.
Simple Animals
Simple animals are the most common Living mob type. Animals have simple AI behaviour if not controlled by a player; depending on the animal, it can be aggressive or passive (sometimes passive-aggressive in the case of Poly).
Notable Simple Animals:
AI
What separates Animals from basic Living mobs is their ability to act on their own. Usually this is limited to chasing and attacking their prey.
Bots
Bots are a special kind of Simple Animal that acts following a specific AI, related to its task. Officer Beepsky, Cleanbots, MULEbots, and so on are classified as bots.
Interface
Bots have an interface that can be accessed by players. They can be hacked or emagged.
Drones
Always player-controlled, drones have a few features from more complex mobs: hands and inventory. They can pick up, drop, and use items, wear hats and backpacks, and are pretty much humans-lite.
Brains/MMIs
Brains are an odd mob-object combination. While they work as object is almost every aspect, they hold a "mob" inside, that can be revived or made able to communicate by using an MMI. Attacking the brain will damage and kill it.
Other
Many unique creatures are also classified as Living. Constructs and shades, for example, are Living, Blobbernauts and blob zombies, bees, and so on. You can generally recognize a Living mob by the single-type health pool and their immunity to stuns and chemicals.
Carbons
Carbons are mobs that approach human-grade mechanics, but not quite. It's a mob type used for complex living entities.
Health
Carbons can take several types of damage, the same way humans can. To heal damage they need to heal the particular type of damage that was taken. Losing enough health incurs a speed penalty.
Status Effects
Carbons are generally vulnerable to most status effects.
Intents
Carbons are able to pick their intent to change their interaction type.
Reagents
Carbons interact with chemicals and is affected by most of them.
Interaction
Carbons usually have hands, which means they can pick up and use items.
Monkeys
Monkeys are commonly used as test subjects or pets on the station. Because of this, they are even closer to humans in regards to mechanics.
DNA
Monkeys have DNA and relative mutations, which can be changed with Genetics.
Inventory
Monkeys have a limited inventory, and can equip backpacks and a few clothing items.
Xenomorphs
Xenomorphs are defined as carbons and share the common characteristics. While they do have hands, they can only use them to pick up facehuggers.
Silicons
Branching out from Living mobs there are also Silicons. This category is composed of Cyborgs and the AI, each of which has their own special mechanics.
Health
Silicons only take Brute and Burn damage, and are immune to other types. The AI can also take pseudo-oxygen damage when losing power. Losing health does not slow them down, but it will lock Cyborg modules.
Status Effects
Silicons are immune to most status effects; they can be stunned through EMPs or flashes, and cyborgs can be locked down to prevent them from acting.
Interaction
Silicons do not have hands. Cyborgs have a set of tools that can be used to interact with other things, but generally cannot pick up items. AIs can interact with electronics but has no physical power otherwise.
Batteries
Silicons have limited energy, and are powerless when out of energy. Cyborgs must recharge every once in a while, and AIs will die slowly if left in an unpowered room.
Laws
Silicons are bound by laws, which they must follow with descending priority.
Humans
The main characters of the game. Despite the name, this category includes all humanoids.
DNA
Humans are defined in appearance by their DNA. Bleeding or touching objects can leave DNA traces reconducible to whoever left them. Additionally, the DNA can be modified through Genetics to alter the human's appearance or to induce mutations.
Species
Humans can be of different species, each with its own traits. Only a few can be chosen at roundstart, but there are more, obtainable in various ways during the game.