Gin is a distillate that's dominated by the Juniper berry. But then there are many other botanicals that the distiller can add to enhance the flavors.
The London Dry is a specific style of English gin. It's a style of gin dating back to the 18th century that shows the botanicals, but what's unique about London Dry Gin is that all of the flavoring must be added during or before the distillation process. If it's called London Dry, the distiller cannot add any further colorants or flavorants after distillation.
Distilled gin is made essentially the same way as London Dry, with one big difference — flavoring can be added after distillation. Compound gin (sometimes called "just plain gin") is a low-priced alternative that doesn't have many rules — and doesn't have juniper flavor until it's added after distillation.
If you enjoyed this video, check out these additional Quick Pours: