Todd Ianuzzi is a violent hoodlum in Highland, age unknown -he's said to have dropped out of Highland High two years before the episode Steamroller but fandom usually assumes he's an adult. (He also turned up at Highland High a few times in early issues of the comics by mistake.)
Beavis and Butt-head look up to Todd and aspire to become part of his gang which consists of himself, his friends Slade and Earl (in the comics), and various minor goons in turf wars against an enemy called Leroy. Todd, however, expresses only contempt for the two, assaulting them with little or no provocation; he will sometimes exploit their gullibility (which can backfire on him). The lads are too thick to notice this and consider it "cool" when he kicks their asses.
He has a girlfriend called Gina. He also, worryingly, is often seen in media dating another Gina, one of Daria Morgendorffer's classmates...
In canon, Daria had no interaction with him (except for him damaging the family's mailbox in Beavis and Butt-head issue 10 during a Halloween rampage) though she did go to school with Gina and Earl.
Fandom has long exploited this link between Daria and a hyperviolent criminal; various fanworks have her running afoul of him in Highland, use him as a threat later on, and in the case of "Of All The People That Won't Be Missed" he's a serial killer dating Jane in a Natural Born Killers pastiche.
Fics include "Till Death Do Us Part" by The Angst Guy, "Daria 2: The Curse of the Misery Chick" (as "Tood Boole") by Aaron Solomon (ben Saul Joseph) Adelman and Barry Eshkol Adelman, "Coming of Age" (a B&B fic crossing over with Daria) by Deep Metal, "Triumph of the Retart" by Peter Guerin, "Of All The People That Won't Be Missed" by LongSnakeMoan, and in the background to Highland set stories by Richard Lobinske.
He was based on a local bully that terrorised Mike Judge and his brother as kids. The idea of a character who was idolised by B&B even as he brutalised them was suggested by writers Sam Johnson and Chris Marcil, who would later write for Daria.
His surname was revealed in This Book Sucks, written by Johnson and Marcil.