The UK Band This was the first band to be made legal (in 1981), and the majority of users still spend most of their time on this band. The majority of old CB sets in use have 40 channels on the UK band.
Channel 19 is the mobile calling channel, where you can get traffic reports and radio reports (to find out if everything is working ok) along with general conversation, although some of the language can get a bit ripe!
Channel 9 is designated as the emergency channel, although many areas no longer have an active monitoring organisation.
Channel 14 used to be the calling channel for CB users at home, and although its still used in some parts of the country, many homebase CBers now use channel 19.
The EU or "European" Band This band is known under a number of names... EU Band, European Band, CEPT, "The Mids" - they all mean the same. It is common throughout Europe.
This band usually has fewer people on it, so attracts those who prefer to have a more private channel (as private as it can be, on a public radio service !). Those wanting fewer interruptions often use this band - event marshals, or a laning convoy are less likely to have anyone come up on the channel, wondering what you are doing !
The only draw back is that the channels are shared with the rest of Europe, which can mean some interesting long distance contacts, but also a certain amount of interference from foreign stations. This interference varies within short and long term weather cycles, getting worse on sunny summer days, and is affected by the 11 year sunspot cycle which peaked in 2001, so is on the decline now but can still occasionally cause a few problems.