A staple of Tex-Mex cuisine, fajitas consist of piling ingredients into a flour or corn tortilla, rolling it up and eating it (usually with just your hands).

Marinated skirt steak is the classic base for fajitas. The meat is cut into strips and cooked over a hot grill (or grill pan). At restaurants, the beef and accompanying vegetables are often brought to the table on a sizzling hot platter for dramatic effect. Guacamole, sour cream and salsa are classic garnishes. Some restaurants serve "fajitas" with beans and rice, but this technically tips them into burrito territory.

The word fajita is a diminutive of the Spanish word for belt and was used to refer to the long, flat (belt-like) diaphragm muscle of a cow (ie, the skirt steak.)

It's said that grilled skirt steak eaten with condiments was first popular with ranch hands along the Texas-Mexican border in the 1930s. Sonny "The Falafel King" Falcon first made fajitas famous in the Southwest starting in 1969. Their popularity has spread in the years since then. Despite the sniffs of purists, fajitas are also made with grilled chicken, shrimp, pork or vegetables.

Reference: What is skirt steak? (Cookthink)
Reference: What is carne asada? (Cookthink)