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Point Entities are entities created at a specific position point on the grid. Each has no area of effect of its own, unless it is given a model.
Prop Entities, available only in the Source SDK, are (usually) non-living inanimate objects that use a model for their appearance and function. They can be static, dynamic, or physics. Some more specific props can even have functionality, such as a prop_door_rotating.
NPC entities, or "Non-Player Characters" entities, are computer controlled characters and creatures that can interact with the player(s) with their own special Artificial Intelligence, or AI. NPCs generally fall into two categories, Friendly, NPCs that help, assist, or do not affect the player, and Hostile, NPCs that attack or negatively affect the player.