Which is shown by this icon to the right. It will upload all of your local changes to the central model. This is important because if you don't do it, all your work will be lost! However, if you are working on something (dims, takeoffs, or an idea that didn't pan out) you can choose to not sync, in which case your changes will not make it to the central model.
This will make sure all changes get to and from the central model to your local file.
This will make sure you don't retain ownership of any elements which would make them so that others couldn't edit them.
Which is shown by this icon (duh). It will save your local file. This is fine to do but not sufficient because it won't get your changes into the central model. All this will do is save your changes to your computer, no one else will see them.