Fertility-F-plasmids, which contain tra-genes. They are capable of conjugation (transfer of genetic material between bacteria which are touching).
Resistance-(R)plasmids, which contain genes that can build a resistance against antibiotics or poisons and help bacteria produce pili. Historically known as R-factors, before the nature of plasmids was understood.
Col-plasmids, which contain genes that code for (determine the production of) bacteriocins, proteins that can kill other bacteria.
Degradative plasmids, which enable the digestion of unusual substances, e.g., toluene or salicylic acid.
Virulence plasmids, which turn the bacterium into a pathogen (one that causes disease).