Pimm's No. 1 Cup is a gin-based fruit cup flavored with fruits and spices. It gained initial popularity in England in the first half the the 19th Century. You can think of it as a gin cut with a citrus/tonic flavor profile.
You will find Pimm's No. 1 Cup is most often drunk in a tall glass filled with ice and topped with English lemonade or ginger ale (sometimes lemon-lime soda or club soda are used). This drink is then decorated with fruit in season.