An old fashioned glass or tumbler is a shorter sturdy, straight sided glass with a heavy bottom that holda from 8 to 12 ounces. This type of glass is perfect to use for drinks that you serve over ice. If you ask for a whiskey on the rocks, then this is the type of glass you most likely will get.
It sometimes can be difficult to find a classic old fashioned glass of good quality. This glass should feel heavy in your hand. It's a very sturdy glass, and the bottom should be quite thick and heavy. You want the sides to go straight up, most glasses today feature a slight curve, but that is not the traditional way these glasses should be designed.
These glasses are in fact very versatile. Of course you could serve practically any mixed drink in them as well as a straight drinks over ice.