Downloadable content (commonly referred to as DLC) is a form of digital media distributed through the Internet. The phrase is used to refer specifically to content created for video games that is released separately from the main video game release.
A patch is a piece of software designed to fix problems with, or update a computer program or its supporting data. This includes fixing security vulnerabilities and other bugs, and improving the usability or performance. Though meant to fix problems, poorly designed patches can sometimes introduce new problems.
This form of DLC is an optional process that adds new content to the game. This could be in the form of additional multiplayer maps; new game modes or even extra campaign missions. Some patches can also add new content to games without any cost, such as the addition of the No Hope difficulty to Resident Evil 6.
This is content like avatar items or dashboard themes that is only available on console platforms. Though owning the game is not necessary to get these in most cases, some situations require you to have the game one way or the other, such as to obtain items tied to the pre-order of the game, or to get content that is only unlocked when the platinum trophy is unlocked.
Just like consoles, PC also gets exclusive content such as wallpaper packs. Some collaborations and DLC content are also only available in the PC versions of the game, such as the Resident Evil 6 ✕ Left 4 Dead 2 collab where L4D characters were added to Resident Evil 6, but only in the PC version.