If a loaded modlist contains mods you do not have installed, but are available on the workshop, you can quickly install the mod by clicking the button in the mod details. To delete a mod list (only the list, not the actual mods), click the mod list button, select the list to be deleted, and then choose the delete option. Any mods that couldn't be matched to your current list of mods will be greyed out. To load a modlist, click the mod list button, then select the save game or mod list you want to load mods from. You will have to choose a unique name, and the mod list will be stored in a folder next to your save games. You can create mod lists by clicking the mod list button underneath your active mods, and selecting 'save current list'.
Note: You may want to occasionally delete obsolete local copies, as having many mods in the mod folder will make RimWorld take longer to start, and the Mod Manager window take longer to open.
Local mods are created in the RimWorld/Mods folder, and can be manually deleted, or deleted by clicking the corresponding button underneath the mod details. Doing this will make sure any updates to the steam version will not break your game in progress. You can make a local copy of any steam mods (or all active steam mods at once) by clicking the corresponding button underneath the mod details (or active mods list). See the documentation for details on how to implement this.