Captain Jack Harkness is human from the 51st century who is a former model turned former Time Agent turned con man when he first meets The Doctor in WWII London. After helping the Doctor and Rose save the day, Jack joins the TARDIS team until his death. Brought back to life (permanently) by a Time Vortex-y Rose, Jack goes on many other adventures (detailed in Torchwood) before running across the Doctor and Martha. Jack leaves the TARDIS a second time (more Torchwood), before returning to help the Doctor and Donna save the universe and leaving the TARDIS for the third and, so far, final time.
A spin off of Doctor Who, Torchwood follows Captain Jack Harkness (John Barrowman) and his team of specialty agents Gwen (Eve Myles), Toshiko (Naoko Mori), Ianto (Gareth David-Lloyd), and Dr. Owen Harper (Burn Gorman) as they search for and destroy all of the things that the government either doesn’t understand or even realize exists. Torchwood Institute had it’s origins outlined in the Doctor Who episode “Tooth and Claw”. Essentially they are viewed as a “special ops” group that acts outside of the government and law.