Collection Vs Environment

[loranger]

Can someone explain the differences between Collection and Environment, please?

I'd like to understand the pros and cons, especially as it looks like you can't re-use variables in queries: I've moved my API key from environment to collection parameters and my authentication variable remains red

[mdreano]

I see collections as a set of endpoints. (one collection per application for me)
Environments are a set of variables i use when calling those endpoints (including some part of the endpoint path)