Radial and ring circuit wiring
This page covers a wiring distinction specific to the UK. While radial circuits are the type used everywhere else in the world, in Britain ring ones are the default, and only recently have they started falling out of favour in new installations.
Radial circuits
Radial circuits are quite simple: the wire connected to a breaker (or fuse holder) is connected to each socket, as required. This doesn't have to happen with a continuous cable: junction boxes are used to branch off into multiple ones heading in different directions.
The amperage of a radial circuit tends to be 16 (15 in North America) or 20A; as such, multiple circuits are required in an installation to reduce the likelyhood of tripping a breaker.
Ring circuits
As the name suggests, ring circuits use a single loop of wire which connects to each socket and then returns back to the breaker. This means that the current can take two paths, thus effectively doubling the capacity of a wire (though real-life conditions can affect this, so a safety margin is present); a 2.5 mm2 conductor can thus be used for a 32A circuit, when on a radial circuit a 4.0 mm2 one would have to be used.
As ring circuits always use a 32A breaker (or 30A fuse), some additional precautions have to be taken to prevent the cord of an appliance to be overloaded in the case of a fault. Fused plugs are used, while hard-wired devices have to be connected to a fused connection unit. Additionally, adding additional sockets to a circuit is more complex, either requiring extending the ring or using a "spur", which is limited to a single outlet.
Their main disadvantage is if the ring is broken at any point - in that case, all the current has to travel through a single cable, which can be easily overloaded and risk causing a fire. Because of this, special in-depth testing is required when installing or modifying a ring circuit, which isn't necessary on a radial one.
Ring circuits became popular in the UK after World War 2, alongside the introduction of the BS 1363 plug standard. During the war, the commission responsible for the future reconstruction efforts predicted a possible shortage of copper. The standard up to that point was to use 10 or 15A radial circuits, usually with a single fuse for each socket in a building, which was quite costly. Comparatively, a single 30A ring circuit could loop around the entire house and connect to each socket, saving money.
While this made sense at the time, ring circuits are significantly outdated nowadays, and yet continue to be legal to install.