Fluorescent tubes generally have a two-pin fitting at both ends of the tube and generally length goes hand in hand with wattage - the longer the tube the higher the wattage.
The largest size T12 (38mm) is no longer being manufactured, however they did use the same G13 cap as the T8 tube which means that you can replace the T12 tubes with the more efficient T8 of the same length. See T8 section below for suitable alternatives.
Standard size T8 (26mm) tubes use the G13 fitting with 13mm between the two pins.
The small T5 (16mm) tubes use the G5 fitting with 5mm between the two pins.
The undercabinet T4 (12mm) tubes use the G5 fitting with 5mm between the two pins.
The ultra slim T2 (7mm) tubes use a fitting called a W4.3 with 4.3mm wide push fit fitting.
Although most lamps emit white light, this can vary from a cosy warm white to a cold white according to the colour temperature of the lamp. The following chart shows the code numbers used for some of the most popular types:-
The Colour Rendering Index is an industry standard reference to a bulb on how well it renders colours of objects it is illuminating. Codes range from 0 to 100, the higher the number the more vivid and lively things look under that light. The lower the CRI rating the more ghostly (faded, dead) objects appear.
If you want your items and people to look good, then high CRI is required. High CRI bulbs generally cost more to buy and cost more to operate as they have low lumen output per watt consumed. Low CRI bulbs put out low quality light, but are very cheap to operate as they have high lumen output per watt consumed.