API
Whilst you may not know what an API is, you'll certainly know when you need one. If you need two or more applications to share data or talk to each other then you're going to need an API.
An API is a way to transport data between two or more endpoints.
Sometimes it is 'pushed' from one app to another and other times it can be 'sync'd' or pushed in both directions to keep two sides updated.
It's not a sexy piece of tech because there's nothing to see but without it, apps wouldn't be able to communicate & would need relationship counselling.
