USB Type A connectors, officially called Standard-A connectors, are flat and rectangular in shape. Type A is the "original" USB connector and is the most recognizable and commonly used connector.
USB 3.0 Type A connectors are often, but not always, the color blue. USB 2.0 Type A and USB 1.1 Type A connectors are often, but not always, black.
Note: The male USB Type A connector is called the plug and the female connector is called the receptacle but is commonly referred to as the port.
USB Type A Uses
USB Type A ports/receptacles are found on almost any modern computer-like device that can act as a USB host, including of course computers of all kinds including desktops, laptops, netbooks, and most tablets.
USB Type A ports are also found on other computer-like devices like video game consoles (PlayStation, Xbox, Wii, etc.), home audio/video receivers, "smart" televisions, DVRs, streaming players (Roku, etc.), DVD and Blu-ray players, and more.
Most USB Type A plugs are found at one end of many different kinds of USB cables, each designed to connect the host device to some other device that also supports USB, usually via a different USB connector type like Micro-B or Type B.
Some USB devices are so small that the cable isn't necessary. In those cases, a USB Type A plug is integrated directly into the USB device. The common flash drive is a perfect example.
USB Type A Compatibility
The USB Type A connectors outlined in all three USB versions share basically the same form factor. This means that the USB Type A plug from any USB version will fit into the USB Type A receptacle from any other USB version, and vice versa.
That said, there are some significant differences between USB 3.0 Type A connectors and those from USB 2.0 and USB 1.1.
USB 3.0 Type A connectors have nine pins, considerably more than the four pins that make up USB 2.0 and USB 1.1 Type A connectors. These additional pins are used to enable the faster data transfer rate found in USB 3.0 but they are placed in the connectors in a way that does not prevent them from physically working with Type A connectors from the previous USB standards.
See my USB Physical Compatibility Chart for a graphical representation of physical compatibility between USB connectors.
Important: Just because the Type A connector from one USB version fits in the Type A connector from another USB version does not mean that the connected devices will work at the highest speed, or even at all. See USB Compatibility Explained for more information on backward compatibility, maximum speeds, and more.