APIs
An API is a set of rules or protocols that let software applications communicate with each other to exchange data, features and functionality.
The measurable.energy API allows partners and customers to extract information by asking for specific data from the m.e Platform and import it into their data systems. It also allows us to pull data from other systems such as weather, carbon intensity, occupancy data etc.
When used correctly an API is a means to make a system smarter by expanding the context surrounding the data you are looking at.
Documentation around setting up the m.e API can be found here:
measurable.energy Customer API - ReDoc
Webhooks
Webhooks are reverse-direction API calls. The m.e Hub pushes data in a JSON format to another system. The other server waits to receive the data and secret (password)
The current data available to send is:
Ruleset updates
LED Status
There is a simple tool here to test that your webhook is working: 
Webhook.site - Test, transform and automate Web requests and emails
Both APIs and Webhooks setups can be found in the Hub under Settings>APIs>API Keys or Webhooks