Thomas Riker is a transporter clone of William Riker before he was serving on the USS Enterprise-D. He was created on a very hostile planet during an attempt to beam up William Riker in the narrow window that the planet's conditions allowed for transport. Dropped back onto the planet, W. Thomas Riker live there assuming that his shipmates had thought he was dead. He was rescued by the USS Enterprise ironically by William Riker. He went on to serve minor roles on other ships in the Federation fleet.
Joining the Maquis
Disillusioned by his life in his own shadow, Thomas Riker used the fact he is basically an exact copy of William Riker to gain access to the starship USS Defiant. Once on board he kidnapped Kira Nerys and faking a catastrophic system malfunction stole the Defiant and took it right into the Cardassian DMZ to use it in the Maquis cause. He was tracked down and recaptured by Gul Dukat and Benjamin Sisko. When last we know he was interred into a Cardassian prison.