A Hero's Sight Range is a measure of how far it can see around it before being blocked by Fog of War. Sight Range is effectively how many units a hero reveals the Fog of War around it without the aid of allied units, wards and structures. The hero reveals Fog of War in a circle around it, unless its view is obstructed in any way.
In most cases, Sight Range is at its highest during the day, and decreases significantly during the night. Since both Dota Allstars and Dota 2 both start during the day time, the hero's default range is measured by its day-time vision.
A hero's sight range can be obstructed by cliffs, ledges and trees. Note that if something is blocking the hero's vision, fog is not revealed behind the obstruction, hence allowing hiding behind trees to be possible.