The difference between these two, is that devDependencies are modules which are only required during development, while dependencies are modules which are also required at runtime.
To save a dependency as a devDependency on installation we need to do an npm install --save-dev, instead of just an npm install --save.
https://medium.com/@dylanavery720/npmmmm-1-dev-dependencies-dependencies-8931c2583b0c#:~:text=The%20difference%20between%20these%20two,an%20npm%20install%20%2D%2Dsave.