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.
Light colour is also known as colour temperature and is expressed in Kelvin (K). You can choose from:
Extra warm white (1700K - 2700K). Atmospheric light. For places with a relaxing character.
Warm white (>2800K). Neutral and soft light. For common areas such as halls, corridors, foyers.
Cool white (>3400K). Fresh and natural light. Good for focus/concentration in schools and offices
Daylight white (>5000K). Equals natural daylight. Suitable for situations where a lot of light is needed.
Light Output (Lumen)
The colour code is a combination of the colour rendering index (CRI) and the colour temperature (K). The first number refers to the CRI (1 = very poor colour rendering, 9 = maximum accuracy). The second and third numbers refer to the colour temperature. For example, a light with a colour code of 830 has very good colour rendering and produces a warm white light.