The cap/base of a bulb is the part that connects it to the socket in the fixture. E27 (Edison screw) and E24 (small Edison screw) caps/bases are among the most popular, although this depends on your location. GU10 is generally the most common cap/base for spotlights. Check which cap/base you need carefully as many other types are also available.
Luminous Flux (Lumen)
Lumen (lm) is the term used to describe the total amount of visible light emitted by a light source. The higher the number of lumens, the brighter it is.