One single piece of glass usually inside of a wood or aluminum frame. Single Pane Glass in sliding glass doors, windows and picture windows. There are also laws in effect that require glass to be tempered for safety reasons. This could be in both single and dual pane glass.
Double Pane: Double-pane windows come with two panes of glass. These panes are separated from each other by a spaced filled with air. These dual pane units commonly referred to as IG's can be found in wood, aluminum and new vinyl windows and doors.