Entity
A object defined within the Source Engine as having characteristics which differentiate it from "the world" (world brushes).
There are three main types of entities:
Brush entities
These entities are built from brushes and covers its area of effect. They include things such as triggers, conveyor belts, and rain.
Point Entities
These entities are created at a position point in the grid, and has no area of effect of its own, unless it is given a model.
Logic entities
These entities are invisible entities that affect the game, including AI managers, lights, and choreography.
Prop entities
These entities are mostly non-living objects that uses a model for its appearance and function. They can be static, dynamic, or physics. Some props can have AI, such as npc_rollermine, and some props can have functionality, such as prop_vehicle.
NPC entities
NPC entities are "Non-Player Characters"; they refers to all of the bots (living/robotic opponents and allies beside the player(s)). They have predefined behavior AI of their own; visible NPCs have their own models too.
Nodes
These entities are created like point entities, and will together create a network that will aid the NPCs (See below.) to gain knowledge about their surroundings, making them smarter than their basic programming.