Light bulbs come in a variety of shapes and sizes and are generally categorized by their bases (the part that screws into a light socket). The letter E before a number indicates it’s an Edison bulb.
The designation Exx refers to the diameter in millimeters, even in the U.S., where the bulb glass is listed in eighths of an inch. (For example, E12 has a diameter of 12 mm) There are 4 common sizes of screw-in sockets used for line-voltage lamps:
Candelabra: E12 (diameter of 12 mm)
Intermediate: E17 (diameter of 17 mm)
Medium or Standard: E26 (diameter of 26 mm)
Mogul: E39 (diameter of 39 mm)
E26 is the size of most light bulbs used in the U.S. It’s referred to as having a “medium” or “standard” base.
E12 is the smaller “candelabra” base. It’s used for nightlight bulbs, and sometimes for decorative light bulbs used in chandeliers and over bathroom mirrors.
E17, “intermediate” base, is in between these two sizes. It’s sometimes used for desk lamps and appliance bulbs, but is not as common.
The large E39 “mogul” base is used on street lights, and high-wattage lamps (such as a 100/200/300 Watt three-way). Chances are you won’t have a need for these guys.