sourceserver.info

Quoted

Q: What’s the difference between a user id, a client index, an edict/entity index, and a CBaseEntity?
A: A CBaseEntity is networked via an entity – and every entity has an index. For the purposes of SourceMod, a CBaseEntity, entity, edict, and edict index are all the same thing. Additionally, a client index is also an entity index in SourceMod. In some places in the Source engine, a client index is the entity index minus one — but that technicality is discarded for simplicity. Lastly, a user id is a unique, 2 byte integer assigned to every user who enters the server. It increases by one for each player that joins.

Quoted

This has actually changed since that blog post was written.

An entity is only understood by the game code, an edict is the engines representation of an entity (and used to network data about the entity to players).

All edicts should have a matching entity, but some entities can exist without an edict. Logic style entities that don't have a client side entity won't have an edict, as they never get networked and exist purely for server side logic purposes.