Model entity
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Entities that use studio model. Every entity using a studio model is almost always a subclass of CBaseAnimating. In FGD they will be defined by @PointClass (or @NPCClass in case of NPCs) and thus in Hammer created via Hammer Entity Tool.
General types of model entities
Prop entities
Prop Entities, are (usually) 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. Examples of common prop entities include prop_static, prop_dynamic, prop_physics and prop_ragdoll.
NPC entities
NPC entities, or "Non-Player Characters" entities, are computer controlled characters and creatures that can interact with the player(s) using their AI coding. 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. Use ai_relationship to change this.
Weapon entities
Weapon entities represent the various weapons available in the game world that players or NPCs can use. These entities define the behavior, appearance, and functionality of weapons such as guns or melee weapons.
Weapon entities handle actions like firing, reloading, and damage calculation. Examples include weapon_pistol, weapon_smg, or weapon_frag.
They usually have a weapon script in which world model and view model is specified. World model being used when the weapon is lying on the ground. When held by a player the weapon usually becomes invisible and the viewmodel entity used in the given game draws the specified view model of the given weapon.
Other pick-able items
For example ammunition items.
has weapon spawn entities which are the weapons that spawn on the ground. When grabbed they give the player a particular weapon entity. For example weapon_smg_spawn gives the player weapon_smg.
Projectiles
Thrown grenades, special attacks by some enemies or shooting certain weapons will result in creation of projectiles.
Examples: molotov_projectile, spitter_projectile, tank_rock, decoy_projectile, tf_projectile_syringe, grenade_launcher_projectile