The Impossible Whopper is otherwise the same as a regular Whopper with tomatoes, lettuce, mayo, ketchup, pickles, and white onions on a toasted sesame seed bun. It can be found during the test at 59 Burger King restaurants in the St. Louis area.
Out of the three largest fast food burger chains (McDonald's, Wendy's, and Burger King), Burger King is the only one that currently offers a vegetarian-friendly burger--the MorningStar Veggie Burger, which basically includes the same components as a Whopper but with a MorningStar veggie patty in place of a flame-grilled beef one. If the Impossible Burger test goes well, it seems likely to replace the current Veggie Burger.
Photo via Burger King.