Electrum server is the silent workhorse powering many bitcoin apps, linking the app with the bitcoin network.
Electrum has two interfaces: the more powerful TCP-based RPC, and a web-frendly REST (esplora). The RPC is more powerful, e.g. it has notifications support.
Interestingly, some operations are easier with the REST, e.g. getting the confirmation height of a transaction is not supported in the RPC API (at least in electrs).
Small differences like this make the life of a programmer fun (miserable?)
