There are two main types of glass tile that are commonly used when making mosaics (although technically smalti could be considered a type of glass tile...more information on that on the smalti tile page!).
Vitreous Glass Tile
Vitreous glass tiles are usually 20mm (1 inch) square with a flat upper surface and bevelled underneath. They are relatively cheap and are heat and frost resistant which makes them ideal for both indoor and outdoor mosaic use.
Vitreous glass tiles are thin, flat and easy to use and cut with tile nippers. They are good for walls, ceilings and three dimensional objects, but are not strong enough for floors.
Vitreous glass tiles are manufactured with intensely bright colors and tend to be more expensive than the muted soft, grainy colors that contain more sand. Some suppliers will sell them loose and in smaller quantities. When cutting the tiles for a particular pattern or design you must always allow for wastage.
Colored Glass Tile
Colored glass tiles are obtained by painting layers of transparent color with a suitable color. You can buy these tiles or attempt to make them yourself.
Glass Tile Suppliers
DickBick Art Materials supplies a great range of metallic vitreous glass tiles that have a fantastic texture and are easy to work with. They also have a range of Venetian glass tiles in a wide range of colors that are suitable for most projects.
Alternatively, MisterArt supplies a wide range of vitreous glass tiles designed for easy cutting, nipping and application that can be used on almost any reasonably smooth surface.
For something a bit different, check out these stained glass tile bags which feature an unusual array of shapes and colours.
If you want to create a mosaic using tiles with a traditional smooth glass surface make sure to look at this glass tile assortment. Alternatively, this glass chip tile package has enough variety to cover most projects.
Finally, if you prefer to cut and shape your own glass tiles from scratch then you could try using these 12"x12" stained glass sheets that give you all the freedom you need to create practically any shape you desire.