Define and document project domain terms and concepts

[mscottford]

We have created a bunch of different terms and concepts in this project, but we're not always using them correctly. We also don't have a place for new folks to look to get up-to-speed with the project. Documenting these things in a .md file in the docs/ directory would help with that.

Once we have these terms defined, we can also open up separate issues wherever we see places in the code where the wrong names are being used.

[mscottford]

This issue was inspired by a recent Twitter exchange: https://twitter.com/Felienne/status/1580870136247902210