It's important to remember that ZDoom is just an engine. In other words, this isn't the game itself. Instead, it's capable of running all of the games that use the Doom engine, like Ultimate Doom, Doom 2, Heretic, and Hexen, so long as you have their original files.
Some of the most noteworthy improvements include support for higher resolutions, improved and expanded textures, free look (look up/down), and translucent objects.
ZDoom is the best way to enjoy Doom. Whether you want to start a single-player campaign or jump into a multiplayer game with up to seven other players, ZDoom should be your go-to.