It's basically a 3-cheese blend quesadilla rolled into a burrito with a filling of seasoned ground beef, Latin rice, reduced fat sour cream, and nacho cheese.
The beef can be substituted with chicken or steak and the test prices range from $1.99 to $2.99 (depending on whether you go for ground beef, chicken, or steak).
It was actually tested last year at a Taco Bell restaurant here in Southern California, but I missed it by all of one day when I went to check it out.
The Quesarito is probably best known as a Chipotle "secret menu" item (although it's hit or miss whether you can get one depending on your local Chipotle). That being said, the idea isn't a new one for Taco Bell...
Some of you might remember the Beefy 5-Layer Burrito back in 2010. Similar to the Quesarito (but much more generically named), it also had two flour tortillas sandwiching a layer of cheese as the wrap for a burrito, but the cheese was warm nacho cheese rather than shredded cheddar, mozzarella, and jack. Also, it was only 89 cents(!).