Mods
Mods are community-built extensions and modifications of the base Space Engineers game. They can add a variety of new blocks, textures, and concepts, though many merely adapt or tweak current content.
Most mods are deployed through the Steam workshop. By subscribing to a mod, your client will automatically have it available to use when hosting a game, and updates will automatically be downloaded. If you are connecting to a server that has mods, there is no need to subscribe to them before connecting, as they will be downloaded as part of your connection process (it does however speed things up if you have already subscribed).
Mods can also be manually added to the game. Mods added in this way are not limited as much as the Steam-based mods are, and extreme caution must be taken lest a malicious mod perform unwanted actions on your system.
For a not-so-comprehensive list of mods, see the List of Mods page or the Steam workshop page. For more information on how to start modding, see the Modding Guide page.