It is never explicitly stated that he and Elrohir 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 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 the books, the brothers are described as dark-haired, grey-eyed and 'elven-fair'. Both Elladan and Elrohir 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.
In fanfiction, Elladan and Elrohir 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.
Nevertheless, Elladan is a victim of twincest, bad slash, and being used as random Elvish eyecandy by fangirls and badficcers who have actually read the books but haven't learnt anything from the experience. There is also a trend of making the twins into the local expies of Fred and George from Harry Potter, having them pull pranks on their father and the populace of Rivendell, sometimes with the help of a young Estel.