Venom is an alien symbiote that has had numerous hosts including Spider-Man, Eddie Brock, Mac Gargan, Flash Thompson, and in one reality at least, Deadpool.
Deadpool and the current Venom, Flash Thompson are team-mates in the Thunderbolts.
Relationship
As an enemy: That idiot Mac Gargan and Normie's T-Bolts tried to kill me... EPIC FAIL
As a friend: Team-mates in the Thunderbolts with Flash Thompson, Venom's current host, not a huge fan of Captain Hero-Worship, but he has proved to be a total bad-@$$, so I will keep the jock army boy on my "do not kill" list.
Eddie (the speedo thief) Brock: Roomies in some dimension or another, never met in the good ol' 616.
Venom/Deadpool: What If?... #1 cover
Deadpool was bonded to the Venom Symbiote in the alternate reality of Earth-90211 as seen in : Venom/Deadpool: What If? #1 and begins to become known as Venompool.