What is USB?
Edits In Progress
Do you need to know more about Universal Serial Bus (USB)?
This tip will provide an overview of both the connectors snd the protocols.
USB is a common mechanism used for connecting gear together for communications or power, and the USB standards detail both the connectors used, and the protocols used through those connectors.
USB Connectors
USB followed with a letter is a connector. It’s hardware. USB-A, and USB-C are the most common with Apple gear, though USB-B and a number of less common connectors also exist.
USB-C:
USB-A, USB-B, and variants:
In recent times, you will usually want to use USB-C connectors, and lots of gear already has or is adding USB-C due in no small part to European electronics regulations.
USB Protocols
USB followed with a number is a communications protocol version. It’s a communications protocol, and not hardware. USB 1.1, USB 2.0, USB 3.0/3.1/3.2, and USB4 are the usual choices here. In recent times, you will want USB4 gear, or USB 3.x gear, and not older and slower and less-capable USB 2.0 or USB 1.1 gear.
Some USB connectors can also be used for different purposes, in addition to uses with USB protocol connections. USB-A connectors can provide power, and USB-C connectors can (and confusingly) support USB connections, USB power delivery, DisplayPort, and Thunderbolt. What a USB-C connector supports can mean reading the vendor documentation.
USB-C with Thunderbolt can be used for some displays, for connecting to storage arrays, and other hardware needing fast connections.
Which gear has which? Which should I use?
The documentation and the packaging will usually (always?) indicate USB-C or USB-A or whatever the particular device was built to connect with, and the package will usually include the cable necessary.
Most mid-grade and higher (recent) storage gear will have a USB 3.x or USB4 connection with USB-C connection on both ends of the cable, using USB-C on the computer snd USB-C on the device. Or will use USB-C and Thunderbolt for storage arrays and such.
Newer Apple gear uses USB-C for most everything, including on Mac, iPhone, and iPad. mac usually provides USB protocol connections, and often Thunderbolt; Other newer Apple gear with USB-C might provide USB Power Delivery or Thunderbolt.
Older Apple gear uses USB-A, or uses other and unrelated connectors.
Gear with the older USB-A connectors can be adapted to USB-C connectors using a dongle such as the Apple USB-C to USB-A adapter, or various other adapters and docks.
Port Icons
Older ports and port icons, showing USB-A connectors and older Thunderbolt ports with icons:
Newer ports showing USB-C, and these three USB-C connections with a Thunderbolt icon but (despite having support) no USB icon shown:
Cables
While USB has made the connectors much simpler, the protocols used with that common connector are more involved, and can require different cables.
There are various cables used with USB-C connectors, including charging cables (which also support USB 2.0, which is old and slow), USB cables of three different wattage ranges, and Thunderbolt cables.
Charging cables are the cheapest, and provide charging and low-speed connections. The three wattage tiers of USB cables can do most everything related to USB protocol connections. Thunderbolt cables can do both USB and Thunderbolt, but tend to be more expensive.
USB-C cables often aren’t well labeled, unfortunately.
Assuming a standards-compliant cable (there are unfortunately some cheap junk cables), you won’t damage the gear or the cable with the wrong cable, but you may not have a working connection.
USB Power
Different devices can have different power requirements, and different docks and hubs can be either unpowered, or powered.
USB-A can usually provide 5W, sometimes 10W, and sometimes 12W.
USB power supplies can usually provide 15W power via USB-C, and some can provide substantially more. There are three broad ranges of USB-C power delivery power and cabling available: power supplies and cables capable or 60 watts or less, of 100 watts or less, and of 240 watts or less.
Drawing too much power from a USB-C connector, dock, or hub will mean a device does not work, and can mean the host, dock, or hub connector port will protect itself and shut down.
If you are connecting with an Apple SuperDrive, that CD and DVD optical media drive requires substantial power, and many adapters won’t work with it. The Apple USB-C to USB-A adapter will.
USB Printers
Please don’t. Please get an AirPrint printer.
More Info
Apple power info: Apple USB-C power adapter volts, amps - Apple Community
USB-C: https://en.wikipedia.org/wiki/USB-C
USB-A, USB-B: https://en.wikipedia.org/wiki/USB_hardware#Connectors
Connect and use other storage devices with Mac - Apple Support
Use a power adapter with your Mac - Apple Support
About Apple USB power adapters - Apple Support
Images from Apple, Wikipedia.