What is a Custom Recovery?

Did you ever have to reset your phone after you broke something?
if so, you've probably seen the Recovery screen before, the one which allows resetting the device or installing updates using a PC

In the Android Modding community, it's common practice to replace the default Recovery with something more advanced with more features, which allow flashing Custom ROMs (see What is a Custom ROM?)

Some common Custom Recovery environments include:

• TeamWin Recovery Project
  This is probably the most popular custom recovery there is. Tt features many advanced features, Android Encryption support, and even backup features. Though it also has it's downsides, for example, many people consider TWRP annoying to use due to the UI being entirely XML, and some dislike TWRP for being more than it needs to be, as well as being slow to update at times.
• LineageOS Recovery
  LineageOS's recovery is another popular choice, it's a modified version of the AOSP recovery with some additional features and changes. Mainly, it allows flashing Custom ROMs and resetting some more specific partitions, as well as ADB access.
• AOSP Recovery
  This is Android's default recovery directly from Google. It's quite basic, and needs to be patched before you can install Custom ROMs (disable signature verification)