I see someone already answered your question.
Their burgers have a very unique taste. I'll add that the fries are awesome. Unlike most fast food chains, In-N-Out fries were real potatoes earlier in the day. They do not franchise out so the burger you get in Porter Ranch, CA is exactly like the burger you get in North Las Vegas.
You can get a single or a Double-Double (two patties, two slices of cheese), you can also order a 3X3 or a 4X4.
Also, there is a "secret" menu that is not printed on any official In-N-Out menu...>
http://aht.seriouseats.com/archives...ate-every-single-item-on-the-secret-menu.html
Double-Double Animal Style is how I get mine. A lot of people like Animal Style fries, too.
Everyone should try In-N-Out once.