Captain Jack Harkness is head of Torchwood Three. Originally from the 51st century Jack was a Time Agent. He met the Doctor in 1941 and travelled with him and Rose for a while until a Dalek invasion of Earth. Jack got killed by the Daleks and was brought back to life, because of this Jack is now immortal.
Gwen Cooper was orinally a police officer but now works for Torchwood. She lives with her boyfriend, Rhys Williams, who is a transport manager. He has no idea that she works for Torchwood and fights aliens for a living.
Ianto used to work at Torchwood until it got destroyed during the battle of Canary Wharf. Iantos girlfriend Lisa got converted into a cyberman.
Owen Harper was working at Cardiff A&E when he was recruited to the join Torchwood. He is now second in command of Torchwood Three.
Like Captain Jack, Toshiko has also met the Doctor during an invasion by the Slitheen. She is Torchwoods technical expert.