This is the most basic way to organize your library, into collection folders. Items in your library can live in as many collections as you like. Use this button in Zotero Standalone to add collections to your library:
More on collections from Zotero.
Tagging is an easy way to categorize items by attaching descriptive words to them. You can tag your information with whatever relevant keyword or term you would like associated with that given item. These tags allow you to sift through your information in a personal way, by the categories that you deem relevant.
When you import items via library databases or catalogs, be aware that tags are sometimes automatically imported alongside these items, using information stored in the database. You may want to keep or delete these tags, depending on their relevance and utility.
To add a tag to an item simply select the tags tab 
in the right column and hit the 
button.
More on tags from Zotero.
The Related tab allows you to associate a given record in a library with any other item, whether or not they are in the same collection. Within the Related tab simply click on the Add button, a pop-up screen will show your libraries, pick any item within the same library then click OK. Your items are now related. Some uses for related items are:
More on related from Zotero.