[Service Contracts] What are Entity Metadata objects (ORM / EAV Attributes management)?

[Ddcdom]

In the customer API I see a lot of references to MetaData (and also in the framework). This post from Alan Kent (https://alankent.me/2014/10/31/magento-2-service-contract-patterns/) provides a tiny bit of information about meta data interfaces.

What are they, when would you use them in your own code and how?

[Vinai]

Hi, this is a great topic, thanks for opening the issue! Conceptually it belongs to the ORM or attribute management side of things, so I'll add those keywords to the issue title for my own reference.
But there certainly is some overlap with service contracts.