Pimm's No. 1 Cup is a British liqueur known as a "fruit cup," a typically gin-based style of liqueur that is meant to be mixed with soft drinks to make highball cocktails. Unlike many other liqueurs, Pimm's uses London dry gin as its base spirit (rather than a neutral grain spirit). It has a light, citrusy, subtly herbal character, and goes great in refreshing summer drinks, the most famous of which is the Pimm's Cup.
Pimm's used to produce six different liqueurs, each flavored with a different base spirit, but those products have been almost entirely discontinued. However, that hasn't stopped enterprising bartenders around the world from trying to recreate their own.