When it comes to gambling, there are two strategies available to a country - regulate it, or attempt to ban it. The United Kingdom chose the regulation route and in 2005 passed the Gambling Act.
Please note: This article refers to the now defunct UK white list which was replaced by specific UK licenses in 2014. All online gambling sites are now required to hold a license with the UK Gambling Commission to be legally allowed to accept wagers from UK customers.
The Gambling Act 2005 is enforced by the Gambling Commission and seeks to ensure that all forms of gambling are conducted fairly and openly, whilst protecting children and vulnerable people.
When it comes to Online Gambling, the act requires certain criteria to be met before an online gambling product can be advertised or otherwise promoted in the UK. Such criteria includes providing a link to the website gambleaware.co.uk and discouraging under age gambling by displaying the 18+ logo.
They must also be licenced and regulated in one of the following recognised jurisdictions: EEA countries*, Alderney, Antigua & Barbuda, Gibraltar, Isle of Man and Tasmania. Together these jurisdictions make up the UK White List, and you may often hear Online Gambling sites referred to as being "White Listed".
Of course, there is no rule that states that you as the consumer have to play or bet at a site that is White Listed, but doing so may give you extra peace of mind.
* EEA Countries means: Austria, Belgium, Bulgaria, Cyprus, Czech Republic, Denmark, Estonia, Finland, France, Germany, Greece, Hungary, Iceland, Republic of Ireland, Italy, Latvia, Liechtenstein, Lithuania, Luxembourg, Malta, The Netherlands (note: not the Netherlands Antilles), Norway, Poland, Portugal, Romania, Slovakia, Slovenia, Spain, Sweden, UK.
For those of you who are interested, the actual Act can be found here: http://www.legislation.gov.uk/ukpga/2005/19
Reference to the advertising of online gambling can be found here: http://www.legislation.gov.uk/ukpga/2005/19/part/16
And the following ammendment was made in 2007: http://www.legislation.gov.uk/uksi/2008/2829/regulation/2/made