In Taxi, individual emails (called Versions) are grouped into Mailings, and mailings are grouped into Projects. All of those things use a Template.
Projects can be used to group together streams of email, such as a welcome sequence, the weekly newsletter or emails that you send for a specific sub-brand.
Mailings equate to a given email campaign or a send. For example, a mailing might be week three of your newsletter, or the second email in your welcome sequence.
Versions exist within mailings, and represent an individual version of an email. Different versions tend to have similar content - perhaps with minor changes for different parts of the audience, or the same content translated into different languages. For example, the German version, or the Bristol version. You may also want to use versions to set up A/B split tests.
There is always a master version within a mailing. Many mailings may only have one version, so this will be the master version by default. The benefit of the master version is that all other versions inherit their content from the master version, unless you specifically change a piece of content. That means you can make different versions, then continue to make changes to the master version, and those changes will propagate through to all versions in the mailing.
You can set access for editors at any level, either an entire project, or specific mailings or versions.
A template is an HTML email build that has had Taxi Syntax added to it, in order to make it editable in Taxi.
You can set which template is to be used at any level. You have to set a template at Project level, but you can overwrite that and pick a different template for specific mailings. We recommend against using different templates at version level in almost all cases, though it is technically possible (for example, to show the same content in an R-to-L Arabic template).