Do you stand in front of the lightbulb counter in shops, totally confused as to what size you need and saying to yourself "I just want a light bulb"
Here is a chart that might help you. It came from here:
Obviously, you also need to know whether you need a bayonet or screw fixing. The bayonet has a push and twist action to it, with a locating pin on either side of said base. The most common size being a 22mm diameter - a BC or B22d. The more unusual slimline base, is a SBC or B15d which is 15mm in diameter.
Screw based bulbs also come in two sizes, again from the same site as found here:
The site is a mine of useful information! Hope this is of use to you and have a lovely weekend folks.