Mobs are living, moving game entities. The term "mob" is short for "mobile".
Mobs come to exist by spawning in various ways. Most will spawn naturally at intervals, upon entering an area (for instance entering or exiting a cave) dependent on factors such as day/night conditions and biome, often in groups with mobs of the same variety.
Most mobs are hostile, but already by the time you reach the Desert, you'll probably have encountered the Critter, a harmless being that likes to eat machines and resources. It flees if scared, but occasionally topples you or your vehicle in its eager to retreat.
Mobs are subjected to the same vulnerabilities as the player, meaning that they can get hurt and be killed when stabbed, shot, bombed or similar. Mobs will usually move around according to a pattern until they spot the player - hostile mobs will give chase and try to kill the player, passive mobs will flee when spotted or attacked by the player.
When encountering a particularly strong mob, it's important to figure out their pattern of behavior. This will help defeating it more effectively.
List of mobs
Mobs are listed and classified by their nature below. For more details on a particular mob, click and view their individual page.