While looking through the Bitcoin Github I stumbled across this page about Bitcoin Quality Assurance.

https://github.com/bitcoin/QA/

After some Googling I discovered that as part of Quality Assurance Bitcoin has Unit testing conducted on its code, and Edge cases are considered.

In simple terms what is Unit testing, and what are Edge cases?

Some other terms I don't understand concern writing test plans, writing test scenarios, and critical path testcases?

What do those terms mean, and what are they used for?