Fireplaces, Heating Stoves, Flues and Chimneys - Wood Insert Basics

I want to buy a wood insert for an existing fireplace. I have searched and searched and cannot find any cohesive overview of what these are all about. Some of these threads say things like "you need a gull length chimney liner" to pass code,etc... Is there anywhere that highlights some of the basics, including:

1) What's required (assuming a good, working, existing, masonry fireplace)
Liners? Surround? What these are for, etc..

2) Catalytic or non? What's the difference? I read that one has "future maintenance implications. Why?

Anything else to watch for?

If this information is already here I am sorry in advance, but right now, trying to go to the stores and get detailed answers is a problem because they are so busy they are pushing product and spending less time with customer details in my opinion.

Thanks
Mike