The cap is the part where the bulb is connected to the socket. There are various types such as "E 26 cap," "E 17 cap" and "E 10 cap." The symbols and numbers such as E26 are determined by the standard.
The first E represents "threaded" and the second number represents "cap diameter."
Screw-in is a type that is often found in household light bulbs and is "connected by turning into a socket." For example, E26 is an insertion type and the diameter of the cap is 26 mm, and many household lighting fixtures use the E26 cap.
Similarly, the E17 has a cap size of 17 mm, and there are sizes such as the E10 for glazed bulbs used in electronic work.
Most household lighting fixtures are screw-in type, such as E26 and E17, but there are also insertion-type fixtures, such as BA9S and BA15S.