It's literally an Extra Long Cheeseburger with the minor addition of pickled (they're going with "marinated") jalapenos. Other than that, ordering one gets you the same two flame-grilled beef patties, mayo, American cheese, lettuce, ketchup, white onions, and toasted hoagie roll of the original.
The Extra Long Jalapeno Cheeseburger is available for a limited time at participating Burger Kings and has been added to the current 2 for $5 mix-and-match rotation, which currently also includes the Big King, the Original Chicken Sandwich, and Big Fish Sandwich. Expect it to cost about $3.50 to $4 if you're only buying one.
Nutritional Info - Burger King Extra Long Jalapeno Cheeseburger (224g)
Calories - 590 (from Fat - 310)
Fat - 35g (Saturated Fat - 13g)
Sodium - 1190mg
Carbs - 45g (Sugar - 9g)
Protein - 26g