The ATmega32u4 belongs to the same family of 8-bit microcontrollers as the ATmega328P, but handles USB communication internally. The 328P doesn't know how to talk to a USB cable, so boards that use the 328P need a USB-to-Serial converter for programming.
Controlling its own USB connection makes the 32u4 a bit more flexible than 328P-based boards. You can program it to act like a keyboard, mouse, or MIDI controller when it connects to a computer. 328P-based boards can't do that.
Most code that runs on a 328P Feather will also run on a 32u4 Feather, but you might have to make some minor changes here and there.
The Circuit Playground Classic also uses an ATmega32u4.
The Adalogger has a built-in SD card socket, which makes it perfect for projects where you want to collect data.
The ItsyBitsy 32u4 uses the same microcontroller as a Feather 32u4, but breaks out more GPIO pins than the Feather. It's best for embedded projects where you want the features of the ATmega32u4 microcontroller but need more pins than are available on the Feather.
There are two versions, one that is 8MHz 3.3V and one that is 16MHz 5.0V