It is never explicitly stated that he and Elladan are twins, but they are born in the same year, and since Elves have a gestation period of roughly a year and most of them wait for several years between having children, it is highly improbable that they are not twins. They are also said to be so alike that they are hard to tell apart, and one is never found without the other.
In fanfiction, Elrohir and Elladan do not often turn up. This is largely because they were not in the movies, or rather, only had a cameo. They are seen next to Elrond in the first movie, before the Fellowship of the Ring goes on its journey.
When they DO turn up, it is largely because the ficcer wants some Elvish eye-candy. In the books they are described as dark-haired, grey-eyed and ‘elven-fair’.
In bad slash, Elrohir is frequently involved with Aragorn or Legolas. Often, Elladan comes along for the ride. Sometimes, the brothers are involved with each other, a subject known as twincest, of which Fred and George Weasley are also frequent victims. There is also a trend of making him and Elladan into the 'Pranksters of Rivendell', pulling tricks on the local populace, sometimes with the help of young Aragorn.
Both Elrohir and Elladan are skilled warriors, who hunt Orcs for revenge. See Celebrían – No! Not Celebrian! The character Celebrían. Whew, that was close! – for why they want revenge on the Orcs. Portraying one or both of the brothers as wusses or wimps constitutes a charge.