The very small USB port found on many non-Apple cellphones, tablets and other portable devices is a Micro USB socket. Considerably smaller than USB Type A and B, Micro USB is also half the thickness of Mini USB (see illustration below). Micro USB and all other USB connectors are expected to be superseded by USB Type C. See Mini USB
, USB Type C
, USB 3.0
, USB OTG
The Smallest USB Socket
The cable that plugs into the Micro USB 2.0 socket on this smartphone has a Type A plug at the other end for the computer or AC charger. Micro USB 3.0 sockets accept a 2.0 plug, but not vice versa.
Micro USB 2.0 and 3.0
These are the two Micro-B sockets. A 5-pin USB 2.0 plug will slide into the left side of a 10-pin 3.0 socket but obviously not the other way around.
Two-in-One Micro USB/USB
The common USB Type A and Micro-B can be combined in the same plug by using a moving outer shell.