tl;dr a 21 map expansion for DOOM II with exclusive levels created by renowned mappers in the community, they're pretty fun and it's worth picking up.
Tired of companies releasing shovelware CDs full of random WADs straight off idgames without giving iD a share of the revenue, iD contracted some of the best and most well known mappers in the DOOM community to make maps for iD's own official map pack.
This resulted in the 1995 release Master Levels for DOOM II.
The pack contains 20 single level wads (TEETH.WAD has a secret level, bringing the map count up to 21), almost an entire campaign worth of new levels.
Master Levels comes with a text mode frontend called Doom-It for launching individual wads in the collection.
Some parts of the original release were not included with the Digital version, such as Maximum Doom, which was a collection of 3201 random amateur maps downloaded off idgames. As you might expect these generally suffered in quality and had problems like some maps being DOOM 1 maps that were hastily ported to DOOM 2, which tended to break balance and cause additional problems. Some of the maps weren't even for DOOM but rather Heretic. However some of the maps were originally intended for Master Levels but didn't make it into the collection.
Other parts that weren't included were a poster, and id STUFF, a bunch of encrypted software that could be unlocked via phone order.
My thoughts on DOOM itself and the quality of the release here are the same as my other DOOM review.
Overall the quality of the map pack is great. The extra levels are a lot of fun and freshen up the DOOM experience.
A quick note, if you plan to play these using a modern ZDoom-compatible source port, check out Master Levels Menu Interface for ZDoom
by Zippy, which is a replacement for Doom-It and allows you to launch the levels directly from the game.