when in doubt, Wikipedia to the rescue for the basics. Here's an excerpt from the entry on William Gilpin:
In 1768 Gilpin published his popular Essay on Prints where he defined the picturesque as '"that kind of beauty which is agreeable in a picture" and began to expound his "principles of picturesque beauty", based largely on his knowledge of landscape painting. During the late 1760s and 1770s Gilpin travelled extensively in the summer holidays and applied these principles to the landscapes he saw, committing his thoughts and spontaneous sketches to notebooks. . . .
For Gilpin, both texture and composition were important in a "correctly picturesque" scene. The texture should be "rough", "intricate", "varied", or "broken", without obvious straight lines. The composition should work as a unified whole, incorporating several elements: a dark "foreground" with a "front screen" or "side screens", a brighter middle "distance", and at least one further, less distinctly depicted, "distance". A ruined abbey or castle would add "consequence". A low viewpoint, which tended to emphasise the "sublime", was always preferable to a prospect from on high. While Gilpin allowed that nature was good at producing textures and colours, it was rarely capable of creating the perfect composition. Some extra help from the artist, perhaps in the form of a carefully placed tree, was usually required. . . .
Although he came in for criticism, Gilpin had published at the exactly the right time. Improved road communications and travel restrictions on continental Europe saw an explosion of British domestic tourism in the 1780s and 1790s. Many of these picturesque tourists were intent on sketching, or at least discussing what they saw in terms of landscape painting. Gilpin's works were the ideal companions for this new generation of travellers; they were written specifically for that market and never intended as comprehensive travel guides.