Bart is Bert's twin brother on Sesame Street (EKA: Episode 0687), a traveling salesman by profession. Although they look identical, Bart is the complete opposite of Bert in personality -- he's cheerful, overbearing, and laughs a lot, a similarity shared with Ernie.
Bert once left to visit his brother, inciting Ernie to call him during the night about frivolous details such as being thirsty. Bart also sent Bert a book for his birthday in a 2003 episode, which Bert preferred to read quietly in observance of his special day.
Bart was presumably named after his (and Bert's) Uncle Bart.