In the video game Half-Life and for most Quake type games and their mods an entity is anything that the player can interact with (use, attack, break, open, turn on, pick up, or be killed by).

Entity types for Half-Life include but are not limted to:
"env" - environment additions
These are effects like blood, sparks, and bubbles.
"ammo" - ammo
Simply ammo to be picked up, each weapon has it's own ammo type. (eg. "ammo_rpgclip")
"func" - functions
These interact with players. They are things like doors, ladders, friction, and much much more. These are applied to brushes (shapes) to turn them into something more than just a polygon. (eg. "func_ladder")
"info" - information about another entity
These tell other entities what they can control, where they lead to, and who they belong to. (eg. "info_teleport_destination" tells where a teleporter will drop you off at.)
"item" - items to grab or use
These can be picked up and held or used by the player. They include guns, armor, and flags. (eg. "item_tfgoal" the flag or object needing to be captured in TFC maps)
"light" - lighting effects
lights to see by. You can control color, path, brightness, and angle. (eg. "light")
"monster" - creatures with AI
Not only are these enemies, but also roaches, police, and friends. (eg. "monster_headcrab")
"trigger" - triggers an event
These cause you and your enemies to fall, move, take injury, or do something against your will when you cross them. (eg. "trigger_push")
"weapon" - a weapon to grab
These are the weapons you can pick up. Ammo is useless untill you have something to put it into. (eg. "weapon_snark")
"xen" - entities of the planet xen
Many things you run into on xen only have their own type. It is an assortment of items. (eg. "xen_tree")

All this was written with the help of world craft