It's simply a document in an internationally agreed format that demonstrates your competence to drive. It is not a replacement for your driving licence, you should carry both with you when an IDP is necessary.
Holders of a valid UK issued photo driving licence won't normally need an IDP for driving in the European Union, Switzerland, Norway, Iceland or Liechtenstein. You may, however, need one for these countries if your licence was issued in Gibraltar, Guernsey, Jersey or the Isle of Man. Anywhere else in the world may require them. If you are hiring a car, check with your rental company whether or not you will need to get one.
You may also need an IDP even when the rules of the country do not require them. In 2013 Florida passed and then rescinded a law requiring all non-US drivers to provide an IDP - but it was in place long enough for Car Rental companies to update their terms of rental to require one. So, even though the law has been withdrawn, some rental companies could still insist on an IDP.
There are two types of IDP according to which treaty the country signed up to - the IDP 1949 is the most common. Some countries require the IDP 1968. Bear in mind that,if you are driving through more than one country, you may need more than one type of IDP.
You can apply for an IDP at PayPoint stores. From April 2024 the Post Office no longer issued IDPs.
The IDP costs £5.50. You will need your full driving licence, valid passport and a passport photo.