A quicksave is an unnamed saving of the game initiated by the player (in single-player mode). The "quick" aspect represents the lack of screens to go through, not the speed at which the computer saves the game; once the key for the quicksave is pressed (the 'g' key by default), the game is saved. When used regularly, this can help avoid losing too much playing time should autosaves not be made often enough for the player's tastes.
There is only one slot reserved for quicksaves (slot number 0). This means that a second quicksave will overwrite the first. In order to keep multiple saved games, a manual (named) save through the game menu/options must be used. When selecting a saved game to load, the quicksave is listed near the top of the list of saved games, alongside the autosave, above the named manual saves.