Using Tags vs Projects

Both Projects and Tags go across all modules and are used to organise the information in your Toolbox site. 
Common uses are for branch locations, business units, product lines, service categories, departments, broad topics. There are some differences in behaviour between projects & tags that might make one work better than the other in certain circumstances. 
 'Projects' will be better for things that have an end date (like projects!). Tags would be better for topics (like "Safety" or "Governance").  
In some cases it will work either way and it's just down to a preference on how you access the info.
Don't worry about setting up either of these in the beginning. You can add tags & projects to content as a bulk action later. 

Projects

Projects give you a window into the system without having to reapply the search parameter when switching modules. 
It's good for narrowing the amount of content a user sees in the system to simplify their experience. e.g. only "England" content is relevant to me and I don't want to see anything for "Italy" or "Singapore". Show me everything to do with "phones" and nothing about "fridges". I'm in "engineering" and I don't care about "marketing".
To filter content by Project you have to go to the Projects module and choose the project first. You can only choose one at a time. You can't start in a module and then apply a project filter. So when you're "in" a project, it's more compartmentalised. 
When you're in a Project, you can also apply tags to filter the search results.
You can make a 'Project' inactive, but still access it if you need to. 
You can have lots of projects and the UI will still be usable. You can search the list of projects by keywords and tags.

Tags

Tags are much more like an additional search parameter. You don't get the 'window' effect. When you switch modules, you need to select what tags to apply to the new search in that module. 
You can be working in the Documents (or another) module and quickly apply a tag to filter the search results. You can apply multiple tags to narrow your search results. i.e. find documents that have both "Safety" and "Governance" tags. 
Tags can't be made inactive. The only way to remove a tag is to delete it, and then you'll lose all those links. 
You have to be a 'staff' user in order to add, edit, delete tags. You can't 'follow' a tag to get notifications. 

Lots of tags won't work very well in a UI sense. They all show at the top of the other search options. You can't search the tags themselves.