Although most people are familiar with tropical rainforests, fewer people are familiar with tropical dry forests. Despite this, they are an important tropical biome - according to Leslie Holdridge's definition of dry forests, they may once have covered 42% of the land mass in the tropics. But most people, when you say "tropical dry forest" reply by asking "tropical rainforest?"
So what are dry forests? While Holdridge's system was complicated, in the simplest terms they are forests were potential evapotranspiration exceeded precipitation. In other words, averaged over the year, the amount of rain that falls is less than the total amount of water that could be lost through transpiration and evapouration, if it were available. On average, the vegetation is water stressed. Classic dry forests are dominated by dry-season deciduous trees (trees that drop their leaves in the dry season), and they have five or more months with less than 100 mm of rainfall. There are exceptions - dry forests in Hawai'i don't have that pronounced a dry season, but they are still dry overall. Holdridge's system was a first approximation - there are areas that meet his definition for dry forest which appear and behave like wetter forests, and they there are forests in higher rainfall areas that resemble dry forests in their species composition and ecological function because they grow on steep slopes with thin soils, or because they are subject to desiccating winds.