Types
There are 3 types of connectors, A, B and C.Type-A
Probably the most common one of all three. It is used on almost all flash drives, power adapters, accessories and even speaker. It shape is rectangular and backward compatible for all versions. There are also mini-A and micro-A USB, but peripherals that uses this standard is very rare.
Type-B
USB Type B is less common. It is more squared compared to other types. It is usually used on printers and scanners. 3.0 ports supports backward compatible, but the 3.0 connector won't work in 2.0 port. It also has two smaller versions called mini-B and micro-B, which people can use it to charge small electrical appliances and older phones.
From left to right: Standard-B, Mini-B, Micro-B, Micro-B USB 3.0 and Standard-B USB 3.0.
Standard-B is used to connect large peripherals such as printers or scanners. Mini-B use to connect older portable devices while micro-B use to connect latest portable devices. The 3.0 versions of standard-B and micro-B are just upgraded to a faster speed.
Type-C
USB Type C is the latest standard. It is rarely used on a computer but more to mobile devices for chargers. USB Type C is tiny as it is only one third of an USB Type A. The feature that is most noticeable for Type C is that it do not have specified orientation, so user can plug it without worrying the USB is plugged upside-down.