The cap/base of a bulb is the part that connects it to the socket in the fixture. In the UK, B22 (Bayonet) caps/bases are still the most popular, although E27 (Edison screw) caps/bases are becoming increasingly popular. GU10 is the most common cap/base for spotlights. Check which cap/base you need carefully as many other types are also available.
Colour temperature (Kelvin)
The colour temperature refers to the colour produced by a light. It's measured in Kelvin (K) and ranges from warm white (1700K - 3000K) to daylight white (6500K). Neutral white (3100K - 5000K) is mainly used for general lighting applications.