USB stands for Universal Serial BUS, and is used to refer to which is certainly more used cable standard in the PC world.
It appeared in the mid-90s and was offered as a replacement to the large number of cables needed for the connection of peripherals. Its field of action since then has not stopped increasing and you can now find it in tablets, smartphones, or game consoles.
Apart from transferring information feature that has become predominantly has been the ability to transmit energy. This is last to that increasingly is more normal to see external hard drives and other devices working without even using Chargers.
Wire connection pins are normally different at each end. This was done to avoid overloading due to the current which is transferred. It is true that there are cables that at both ends have the same connectors but are rare.
The best way to meet these standards is reviewing the old.
In particular, we have:
USB 1.0. Sets a speed of 1.5 Megabits/sec enough for mice and keyboards but very slow hard drive.
USB 1.1. We arrived up to 12 Megabit/s, i.e. eight times more rapid.
USB 2.0. 480 megabit/s, a qualitative leap multiply the speed by 40. In laptops, especially former was interesting to go testing each peg until we gave with which offer us the maximum speed that as you can imagine is noticed much.
USB 3.0. 4.8 Gigabit/s, multiply by 10. It has the limitation that many devices are unable to give as much speed or simply have been left with the previous standard.
What does USB 3.1?
Among other USB 3.1 improvements it offers the following improvements that may be desirable:
Speed. USB 3.1 multiplied by two 3.0 speed giving rise to 10 Gigabit per second. Comparing with its only rival, Thunderbolt, is slower than this because in its second version becomes 20 Gigabit/s but a USB cable can give load devices and also Thunderbolt still a single Intel development can buy it it at Apple computers.
Backward compatibility. As others there is backward compatibility provided the connection type is compatible.
Ability to give current. Increases the ability to stream up to 100 Watts. You can even give it in both directions, e.g. a tablet charge a smartphone or on the contrary.
New connector. A new type of connector there are type C which is reversible. No more fighting with the position. Smaller than which we are accustomed.
However in this case we lose compatibility with previous connections.