Tammy is a former resident of the Sunnyvale Trailer Park, Ricky's mother, and the former wife of Ray.
Tammy and Ray had gotten together sometime in the past and bought a trailer together in the Sunnyvale Trailer Park, which Ray had continued to live in and pay for years after she was no longer around ("The Shit Puppets", "You Got To Blame The Thing Up Here"). Around this time, Ray had taken up drinking vodka by putting it in water bottles in an attempt to fool Tammy into thinking he was consuming large amounts of water ("Jim Lahey Is A Fuckin' Drunk And He Always Will Be"). Long after she had departed from Ricky and Ray's lives, Ray continued to think of her as a blessing, and was quick to defend her against negative and opprobrious remarks by others ("I'm Not Gay, I Love Lucy... Wait A Second, Maybe I Am Gay", "You Got To Blame The Thing Up Here").
While Ray and Ricky continued to hold Tammy in high regard, this sentiment was not shared by other residents of Sunnyvale, with Julian once remarking that she's "fucked in the head" ("You Got To Blame The Thing Up Here").
Background
Tammy's fate is never made explicitly clear, while certainly no longer around as of Christmas 1997, there are conflicting references as to her current/final status. Most references refer to Tammy in the past tense, implying that she had died, however, there are also some made in the present tense, suggesting that she merely left Sunnyvale, leaving Ray and Ricky behind. Either way, this makes her one of several parental figures, such as Bubbles's parents and Julian's Grandmother, that had left the park prior to the series setting (and the prequel, as well). It has also been implied that Tammy is responsible for some of Ricky's mannerisms and fatherhood views that are often looked down upon by other people, such as letting Trinity drive the Shitmobile. Tammy also, a long time ago, had an affair with Mr. Lahey which makes him Ricky's real father and could be part of the reason she left Sunnyvale. This was told during the 11th season.