A USB audio MIDI interface is a unit that allows you to record both audio AND MIDI into your computer (or mobile device, in some cases).
In order to get great sounding audio into your computer, you need a good audio interface. And I talk a lot about those. But what if you want to connect a keyboard to also record MIDI?
Not all audio interface units have MIDI inputs and outputs. So if you want to record BOTH audio AND MIDI, you'll need a USB audio MIDI interface unit. This is assuming you use USB, as most of us do. But if you use something different - like thunderbolt - you'd obviously need an interface specifically for that.
One terrific USB audio MIDI interface unit is the M-Audio M-Track 2X2M. As the name of the model implies, you can plug two mics into it (2X for "2 XLR"). These inputs are combo jacks so they will accept XLR or quarter-inch balanced plugs.
And there is a MIDI input where you can plug in a MIDI keyboard, as well as a MIDI Output.
So with this interface, you can record 2 mics with pristine preamps. And then you can create MIDI tracks to play your virtual instruments in your DAW recording software.
The 2X2M has several other input/outs (I/O). The USB interface is a USB-C type. But it comes with a standard (USB-A) cable and will work with any USB input on your computer.
Then there are 2 quarter-inch instrument inputs that are optimized for guitar and bass, but can accept any instrument input. Also, you have balanced main stereo outputs so you can plug these into monitor speakers or a mixer running your monitors.
This interface, like most decent units, also has phantom power for your condenser mics.
Obviously the M-Audio unit is not the only USB audio MIDI interface out there. But if you want to plug a MIDI keyboard into your interface, you really need to be sure before you buy one that it has that MIDI capability. The interface I usually recommend - the Focusrite Scarlett Solo or Scarlett 2i2 do NOT have any MIDI I/O. The Focusrite Scarlett 2i4 interface, however. DOES have MIDI capability. So make sure you do your research.
One other USB audio MIDI interface is the Steinberg UR22mkII. It is similar to the M-Audio, except the Steinberg can ALSO record into your iPad! It also has what it calls "Loopback" for streaming performances live to the Internet.
As far as I/O it's very similar to the M-Audio. Both units can record at 24-bit/192kHz. The Steinberg does not have 2 separate instrument inputs, but since the mic inputs are combo units, you can plug instruments into those.
So here is short list of interface units with MIDI I/O. Obviously this isn't a list of every unit in the world. But it should give you a good starting point if you need audio AND MIDI recording capability:
M-Audio M-Track 2X2M
Focusrite Scarlett 2i4
Focusrite Clarett 2Pre USB
Mackie Onyx Producer 2.2
IK Multimedia iRig Pro Duo (works with iPhone and iPad also!)
There are lots of others, of course, but this list should give you a good selection to choose from if you are looking for a USB audio MIDI interface.