This plug has three rectangular prongs that form a triangle. British Standard BS 1363 requires use of a three-wire grounded and fused plug for all connections to the power mains (including class II, two-wire appliances). British power outlets incorporate shutters on line and neutral contacts to prevent someone from pushing a foreign object into the socket.
The British domestic electrical system uses a ring circuit in the building which is rated for 32 amps (6 amps for lighting circuits which are usually spurs). Moreover, there is also a fusing in the plug; a cartridge fuse, usually of 3 amps for small appliances like radios etc. and 13 amps for heavy duty appliances such as heaters. Almost everywhere else in the world a spur main system is used. In this system each wall socket, or group of sockets, has a fuse at the main switchboard whereas the plug has none. So if you take some foreign appliance to the UK, you can use an adaptor, but technically it must incorporate the correct value fuse. Most would have a 13 amps one, too big for the computer for example. BS 1363 was published in 1962 and since that time it has gradually replaced the earlier standard plugs and sockets (type D) (BS 546).
British plugs are no doubt the safest in the world, but also the most hulking and cumbersome. That's why people often make fun of them saying that British plugs are mostly bigger than the appliance they're connected to.
Countries Using Type G Electric Plug/Socket - Bahrain, Bangladesh, Belize, Botswana, Brunei, Cameroon, Channel Islands, China, Cyprus, Dominica, El Salvador, Gambia, Ghana, Gibraltar, Grenada, Guatemala, Guyana, Hong Kong, Iraq, Ireland, Isle of Man, Jordan, Kenya, Kuwait, Lebanon, Macau, Malawi, Malaysia, Maldives, Malta, Mauritius, Myanmar, Nigeria, Oman, Qatar, St. Kitts-Nevis, St. Lucia, St. Vincent, Saudi Arabia, Seychelles, Sierra Leone, Singapore, Tanzania, Uganda, United Arab Emirates, United Kingdom, Vietnam, Yemen, Zambia, Zimbabwe.