Pimm's is a gin-based liqueur, of about 25% ABV, and it tastes rather medicinal on its own. But when mixed with "lemonade" (more on this in a moment), it becomes a very basic Pimm's Cup, and is the very perfect thing for a hot summer day. Lemonade, in the British world, is a very dry fizzy drink with actual lemon juice in it. The closest I can come in the US is San Pellegrino Limon, but any of the lemon sodas will do in a pinch. I like to squeeze in some fresh lemon just to cut the inevitable sweetness of an American soda. Pimm's cup may also, incidentally, be made with ginger ale. Or, for a Pimm's Royale, champagne.
Traditionally, a Pimm's Cup is built in a large pitcher, with slices of cucumber and lemon, and topped with borage and mint. I never have been able to find borage, so I use more cucumber, and add orange, lime, and strawberries. So basically, a fruit cocktail in a cocktail. Mix 1 part Pimm's to 3 parts lemonade, add lots of ice, and enjoy, preferably while floating down a river, enjoying a picnic lunch, or overlooking some beautiful slice of the English countryside.
Or whenever. This stuff is great, and 'tis the season.