Jack cheese, often called California Jack or by its regional variations Sonoma Jack and Monterey Jack, is made from cow's milk and generally available unaged (though aged Jack can be found along the west coast of the US and in some specialty shops). It's semi-soft, melts easily, and has a mild and creamy flavor with a gentle bite. Monterey Jack cheese was popularized in the mid-1800s by David Jacks, Monterey land baron.
David Jacks, who was born in Crieff, Scotland, came to America in 1841. The California Gold Rush brought him eventually to Monterey, where he became a wildly successful businessman -- some say, by loaning struggling Mexican families money, and then foreclosing when they couldn't pay -- eventually owning more land in Monterey County than anyone else. Much of it was ranch land, and it's no wonder the local cheese, formerly called "Queso del Pais" (literally, "country cheese"), came to be known first as "Jacks Cheese" and later simply "Monterey Jack". Some suggest that Jacks particular treatment was tinged with a similarity to Dunlop, a supple and moist Scottish cheese.
Queso del Pais came to Monterey via a circuitous route: Romans brought it from Italy to Mallorca, one of the Balearic Islands off the coast of Spain. From there, it was brought to Mexico by Franciscan monks in the 18th century, and eventually made its way north to the Monterey area.
Like Gruyère, however, Monterey Jack cheese is not without controversy around its name. The Monterey County Historical Society has heard from Teresa Russell, granddaughter of Dona Boronda, who also sold cheese in the Monterey area in the mid-1800s. She indicates that "jack" cheese is called such because the vice used to squeeze whey out of the curds while making this cheese was called a "housejack". According to Teresa, Dona Boronda's family was responsible for bringing this cheese recipe from Spain supplied the cheese she made to David Jacks, to sell in a store he owned.