To play doom, you need two things: an executable and an iwad file.
IWAD stands for Internal Wad. If you have one, you can also slide PWADs (Patch Custom Wads) into the source port or run from the executable.
The original executables are DOOM.EXE and DOOM2.EXE and run in the MSDOS Environment, OR Dosbox. You can also play
in a much higher resolution with other perks with a Source Port. A Source Port is a modern executable used to enhance
the old school doom and doom 2 with new more modern ways of playing, such as higher resolutions as mentioned. You can
also use ZDL to launch multiple files at once via any source port, so you can have a patch
wad playing (PWAD) as well as maybe a sprite or model pack (enemies) or even a music collection (like IDDQD by Andrew Hulshult).
To get the IWADS legally and for dirt cheap, visit Steam and sign up if you haven't already (they're literally 4.99 unless on sale) and buy The Ultimate Doom or Doom II: Hell on Earth. If you get those two, you can play most pwads and can witness both beautiful chapters of the classic doom saga created by id themselves. There are other options, however, such as Final Doom (made by TeamTNT and officially licensed as the third doom game) or Master Levels for Doom II (Featuring the best levels made at the time by regular people not in game companies either).
If you get the IWADS this way, you don't need an executable added, because it comes with the standard crappy DOS Version. Keep in mind, those suck now.
Get a source port (free!) and set them up to play from the iwads. for ZDoom, you just put them in the same folder. Most other source ports work the same way.
Then to play a patch wad (just one) slide it into the executable that normally launches the game. If you want more than one, like I said, get ZDL.
NEW DOOM EPISODE 5 FREE! "SIGIL"
John Romero for the 25th Anniversary of DOOM released a now fifth and final episode. You can snag the BOOM-Compatible version here (or other dos version like vanilla) or you can fire up GZDoom with DOOM.WAD and slide over this file instead. Both work, but the DOS version takes over episode 3 for those of you who only play in DOS or only have the 3-episode classic version (for some strange reason).
You can also Purchase the version with the real Buckethead Soundtrack. There are also several Boxed-Sets for high prices on ebay if you are a collector.
SINGLE PLAYER SOURCE PORTS
The most-used and most-enjoyed by the fair majority of the doom community is GZDoom.
GZDoom @ ZDOOM.ORG is the latest version targeting current systems with modern graphics hardware. (Vulkan/OpenGL 4.5 capability recommended, but the minimum requirement for the hardware renderer is OpenGL 3.3, the minimum for the software renderer is Direct3D 9). It also plays Final Doom, Heretic, and Hexen: Beyond Heretic. You can get them all on steam. There are also free iwads such as Chex Quest, Harmony, Hacx, etc.
LZDOOM and the final version of the now defunct ZDOOM are also available from this website.
My second favorite source port is: The Doomsday Engine. It has a lot of enhancements such as 3D Models available for the enemies and actual High-Resolution texture packs you can download and use. It may bog you down slightly but if you have a high speed computer maybe not. It also works with Heretic and Hexen, just like GZDoom.
DEATHMATCH/MULTIPLAYER SOURCE PORTS