(Covers information from Anderson's universe)
The Licker is a biological weapon created by directly injecting the t-Virus into live human tissue. It mutates upon consuming (uninfected) human flesh; in doing so, it will infect the victim with a mutant strain of the virus which also causes them to mutate. This happened to Matt Addison, who was later used by Umbrella in their Nemesis Program.
In Resident Evil, a Licker is released when the Red Queen is shut down temporarily and goes on a rampage, killing Spence (who returns briefly as a Undead) and Chad Kaplan as well as infecting Matt Addison before he and Janus Prospero kill it.Resident Evil: Retribution, a 50 ft mutated Licker, called the Uber-Licker, is seen chasing Alice, Luther, Barry Burton, and Leon Kennedy while they are trying to outrun it in a Rolls Royce Phantom. Alice drops a ton of rocks on it, but it survives and is dispatched by the Red Queen to kill the survivors at the elevator. The Uber-Licker captures Becky, severely injures Barry Burton and kills Good Rain. Alice chases and disables it with shots to the head, but it survives and attacks her and the rescued Becky. The two escape with a grapple gun with Alice dropping a belt of activated grenades near the Uber Licker. The grenades detonate, killing the Licker and destroying the cloning facility it is located in at the time. At the end of the film, two more Uber-Lickers can be briefly seen standing on top of the White House's walls and jump behind it. They were presumably killed soon afterwards.