A quesadilla is a flour or corn tortilla

Tortilla

In Mexico and Central America, a tortilla is a type of thin, unleavened flat bread, made from finely ground maize...

 filled with a savoury mixture containing cheese and other ingredients, then folded in half to form a half-moon shape. This dish originated in Mexico

Mexico

The United Mexican States , commonly known as Mexico , is a federal constitutional republic in North America. It is bordered on the north by the United States; on the south and west by the Pacific Ocean; on the southeast by Guatemala, Belize, and the Caribbean Sea; and on the east by the Gulf of...

, and the name is derived from the Spanish word queso (cheese).
The specific origin for the quesadilla was in colonial Mexico. The quesadilla as a food changed and evolved over many years as people experimented with different variations of it.
A quesadilla is made with a tortilla and is filled primarily with cheese

Cheese

Cheese is a generic term for a diverse group of milk-based food products. Cheese is produced throughout the world in wide-ranging flavors, textures, and forms....

, other ingredients, such as cooked meat, refried beans

Refried beans

Refried beans is a dish of cooked and mashed beans and is a traditional staple of Mexican and Tex-Mex cuisine, although each cuisine has a somewhat different approach when making the dish.-Ingredients and preparation:...

, or vegetables may be added.