Tags are part of the G2G system and are a way to allow you to say what topics are involved with your question. Surnames and projects will have tags as well as other topics. As you said, they are used in email and G2G discussion.
Projects are groups of people who want to manage classes of information (profiles or categories). Projects vary in purpose so you have to look at projects that interest you. You can request to join a project and help out.
Categories are similar to tags in concept but are quite different in implementation. Categories allow putting a profile, space page or other category into a specific "bucket". For example, you can put a location category on a profile and then that profile will show up in the category. There is a button on the profile edit window that will insert a category for you. You can have multiple categories. So, a person can be in the Boston_Massachusetts category and also in Pittsfield_Massachusetts category to indicate born in one and died in the other. There are cemetery location categories, etc.
Stickers are a way to add a bit of window dressing on a profile and indicate something about that person. For example, an ancestor who was a United Empire Localist can have a sticker saying so. Some stickers also add categories but not all. There are also stickers for veterans of various wars, stickers for WikiTree events that you participate in. Lots of different things.
