The lake is about 30 kilometres long and 25 kilometres wide. It is shaped roughly like a fist made with the left hand, with the index finger and thumb extended. The thumb forms Kempenfelt Bay on the west, the wrist Cook's Bay to the south, and the extended finger is Lake Couchiching on the north. Couchiching can be considered a third bay of Simcoe, but the narrows between the two separate them enough to be considered two lakes. The narrows, known as "the place where trees grow over the water" was an important fishing point for the First Nations peoples that lived in the area, and the Mohawk term, toran-ten eventually gave name to Toronto by way of the portage route running south from that point, the Toronto Carrying-Place Trail.
Lake Simcoe contains a large island, Georgina Island, on which there is a First Nations reserve. A number of major rivers of southern Ontario flow, generally north, into the lake, draining almost 3000 square kilometres of land. The Trent-Severn Waterway is the most important river system draining into Lake Simcoe, connecting the lake with the Great Lakes Lake Huron and Lake Ontario (Simcoe itself is not a Great Lake).
The northern shore of the lake contains thousands of cottages, and is one of the most popular vacation areas in Ontario.