I am nothing if not a lemming.
Who is Sam?
In Stargate canon, Sam's a genius of an astrophysicist with a deep and obsessive love of alien technology, almost anything scientific, motorcycles, explosions, and blue Jell-o. She has intentionally blown up both a sun and an alien spaceship. Also, she was taken over by a Goa'uld (the evil alien snake things), but it turned out to be a Tok'ra (one of the mostly less evil alien snake things) instead. When the Tok'ra (named Jolinar) sacrificed her life so Sam could live, she retained the ability to detect other people who had Goa'ulds in them and to occasionally use Goa'uld technology. Alternate universes and aliens are not at all unusual to her. Wormholes are her passion.
Sam is also a US Air Force Major with experience flying F-16s during the Gulf War, level three hand-to-hand training, and lockpicking. She has a bad habit of getting involved with alien men off-world, who then usually die in some kind of emotionally tormenting way. Also, as a person, in times of crisis involving both people and science, she occasionally forgets that people are most important, in her fascination with the science. This can come off as emotionally cold, and probably wouldn't be inaccurate, but that doesn't mean she doesn't care about people--they're just not always her first thought, even if they should be. Also, when faced with emotional crises, her M.O. is to evade and suppress it by burying herself in her work--it's nothing personal.
Her father (Jacob Carter) recently died (May 2005), but she wasn't notified about it by Carter until last September (zombie weekend, to be exact). Her mother died in a car accident when she was a teenager. She has one brother (Mark), a niece, and a nephew, but hasn't seen them since she became mini. At the SGC in Colorado Springs, CO, Hammond and Carter both know she exists, but O'Neill does not.
She's been pulled from canon at a point before she had command, so she's not comfortable with it--she can make scientific miracles happen, but isn't always good at making necessary military decisions. Also, she hasn't been keeping up with her training and her body is weaker than she's used to, so she's not as good hand-to-hand physically as her training might imply.
Who is Sam at Fandom High?
In canon, Sam's in her forties. Here, obviously, she's not. She's been cloned by Loki (that pain in the butt Asgard who also cloned Kawalsky and Janet Fraiser), but unlike the miscalculation that made mini-Jack, well, mini, Sam intended to be this size. Long story short, Eighth (Replicarter) was a human-form replicator of the older Samantha Carter and used Carter (the older one, just to be clear) to make the replicators (little weetiny robots that eat technology) invulnerable to the all-powerful ray gun that O'Neill almost died to get. Carter trusted Eighth (for complicated personal reasons involving O'Neill) and didn't pull the trigger when she should have--leaving the Earth at risk from the replicators. Anyway, Sam chose to be this physical age so that she would be around to stop Eighth, in case the other Carter couldn't. The adjustment hasn't been easy, or smooth, though.
The many mostly dead loves of Samantha Carter:
Jonas Hansen (Human, Ex-fiancee, developed a god complex, deceased)
Narim (Tolan, Sam gave him her cat, presumed deceased)
Martouf (Tok'ra, his symbiote was mated to Jolinar, was brainwashed to become an assassin, deceased)
Joe Faxon (Human, Sam's husband in an alternate timeline, they returned to the timeline and he died)
Pete (Human, Ex-fiancee, police officer, stalker, bought her a house so she dumped him)
Jack O'Neill (Human, commanding officer, out-of-control UST)
FH romantic history:
-John Connor (one date, one motorcycle skeleton, one jeep, and Homecoming)
-Jack O'Neill (disappeared without notice, she broke up with his answering machine)
-Lee Adama (though you can't necessarily call it dating--and he had to leave abruptly because Eighth tried to destroy his galaxy)
-MacGyver (ooh, complicated. He kissed her, then left to return to his old job.)
-Currently dating Jack Harkness, who is also dating Cedric Diggory. So far, so good.
Since coming to Fandom:
-Her best friend has been taken over by a Goa'uld who enslaved a bunch of guys using pheromones
-Helped liberate an X-box and pretended to kiss Six
-Suggested that Sheppard urinate on the snow monsters
-Rescued Lee from Eighth
-Been strangled by Eighth
-Helped kidnap Camulus in his sarcophagus
-Helped with comms during Study Group
-Baked cookies that made a bunch of people think they were characters from Grease (oops)
And if you're still reading this, you deserve a medal.