Tempered glass, also known as safety glass, is the smartest choice for windows and other glass structures in the house. Tempered glass is heated very quickly during the manufacturing process and then cooled down; this rapid change in temperature makes it about four times stronger than untreated glass.
When tempered glass breaks, it is designed to crumble into small, dull pieces. Untreated glass will break under relatively little force and shatter into dangerous shards.
The International Residential Code dictates where tempered glass should be used in a home and where normal glass is acceptable, but keep in mind that there really are no downsides to insisting on safety glass when it is an option, even if it is not required by the code.
If you are like most homeowners and you are considering replacing the windows in your home, your first consideration will likely be frame material. Are you going with wood? How about low maintenance vinyl? Do you opt for composite windows instead?