Stupid question I should know better

What are the differences between all the different terminology on amplifiers and head units when it comes to inputs and outputs. Some amps have speaker level inputs, some have speaker level outputs (I assume all would have to have these) some have "low level inputs/outputs", "line level inputs/outputs", "pre-amp outputs", etc...I know how most of this works, but are their compatibility issues to think about when choosing an amp based on the different options available?

Try this:

https://www.the12volt.com/glossary/glossary.asp