Lees: Sediment—dead yeast cells, grapeseeds, stems, pulp and tartrates (harmless tartaric acid crystals)—remaining in a barrel or tank during and after fermentation. Immediately following fermentation, wine should be racked off of the gross lees, the large particulate matter such as seeds, skins and stems, which are rich in spoilage organisms. The wine may be aged for an extended period on the fine lees, however, in what's called "sur lie" aging. Fine lees, the dead yeast cells leftover from fermentation, can enhance an aging wine with added richness, flavor and aroma complexity, and can also bind with excess tannins.
Leesy: Aromas and flavors resulting from a wine having spent some time on its lees, typically adding rich, creamy and yeasty components to its profile.
Sur Lie: Wines aged sur lie (French for "on the lees") are kept in contact with the dead yeast cells and are not racked or otherwise filtered. This is mainly done for whites, to enrich them. (It is a normal part of fermenting red wine, and so is not noted.) The concept originated in Burgundy, with Chardonnay, but is now popular around the world with numerous white grape varieties. Sur lie aging can be overdone, leading to an off-putting leesy flavor.