Any Scottish peak between 2500 and 3000 feet (a less catchy 762 and 914 metres for you metric fans) is described as a Corbett. J. Rooke Corbett, for whom they are named, spent the 1920s and 30s climbing all the Scottish hills of at least 2000ft, and was the first person to do so. The list of Corbetts was compiled in the process, but only published after his death in 1949. Hills above the 2000ft mark that fail to make the 2500ft grade are known as Grahams, whilst the more famous Munroes are those peaks greater than 3000ft.
Corbetts are considered distinct if there is a 500ft drop between them, by which reckoning there are 219 of them. A Corbetteer is someone who as climbed the complete set - a process known as Corbett bagging. Whilst all the Munroes are found in the Highlands (that is, north of the Highland Boundary Fault), Corbetts are spread throughout Scotland, with peaks to be found as far south as the Galloway Forest Park.The Isles of Skye, Lewis and Mull also feature Corbetts. Whilst this makes it easier to find and climb a Corbett, the clustering of Munroes makes it easier to bag several of those technically harder peaks in a day.
The comprehensive list of Corbetts (and Munroes) is maintained by the Scottish Mountaineering club, with The Corbetts and Other Scottish Hills detailing recommended routes - weather permitting, it's possible to hike all the Corbetts (and all but one of the Munroes) without climbing gear. You can also browse a google map of the Corbetts here.