USB-A
pietz, CC BY-SA 3.0 , via Wikimedia Commons
This is the standard USB connection that most computers offered prior to the introduction of USB Type-C (USB-C). Even after the introduction of USB Type-C, this is still quite common.
It can provide data transfer rates up to the USB 3.1 Gen 2 (10 gbps) specification depending on the host and device, but does not directly support video in the way that USB-C Alternate Mode does. This limitation makes DisplayLink USB graphics adapters and docking stations ideal on systems that do not have USB-C, or in instances where more displays are needed beyond available video outputs of a PC.
USB-B
Fred the Oyster, CC BY-SA 4.0 , via Wikimedia Commons
IngenieroLoco, CC BY-SA 4.0, via Wikimedia Commons
This type of connection comes in a couple different styles depending on whether USB 3.0 and higher transfer rates are supported (bottom graphic). Usually this type of connection is used to plug into USB devices that do not have a fixed cable connected, such as USB docking stations, USB hubs, printers, and others.
USB Mini-B
Fred the Oyster, CC BY-SA 4.0 , via Wikimedia Commons
One of the first connectors for charging a smartphone, wireless game controller (such as the Sixaxis and DualShock 3), and other small devices such as external hard drives. Not commonly used today, but is still used in some cases. Most devices using USB Mini B are using USB 2.0, though a USB 3.0 variant does exist. This specification also added USB On-The-Go (OTG) functionality, though it is more commonly implemented with Micro USB.
USB Micro-B
Fred the Oyster, CC BY-SA 4.0, via Wikimedia Commons
IngenieroLoco, CC BY-SA 4.0 , via Wikimedia Commons
A smaller connector that serves many of the same uses as the Mini B connector, with added optional features such as Mobile High-Definition Link (MHL) to allow devices like smartphones to output video to larger displays without requiring a dedicated port for video output.
The larger variant of USB-B is most commonly used for external hard drives for higher 5Gbps transfer rates.
USB-C, Thunderbolt™ 3, and Thunderbolt™ 4
Niridya , CC0, via Wikimedia Commons
The most recent USB connection, USB Type-C (USB-C), represents a major change in what USB can do. The connector is smaller, can be connected in two orientations, is able to carry substantially more power and data, and can directly carry video signals of multiple types (HDMI, DisplayPort, etc.) Intel has also adapted the USB-C connector for use with Thunderbolt 3 and Thunderbolt 4.
It is important to note that while all Thunderbolt 3 and Thunderbolt 4 connections are USB-C, not all USB-C connections can be used with Thunderbolt 3 or Thunderbolt 4 devices.
More details regarding physical USB connections can be found on Wikipedia . The graphics depicted here are adapted from Wikimedia Commons by various artists under the Creative Commons Attribution-Share Alike 3.0 Unported license.