Stellar is a platform that helps developers in building financial products and applications. It features a decentralized distributed database, Multisig, and smart contracts. Horizon is Stellar's client-facing API server that connects to the ledger (Stellar Core). The Stellar Horizon API is REST based and returns data in JSON format. It is used to submit transactions to Stellar Core, check the status of accounts, subscribe to event streams, among other functions.
the API/code can be acquired at https://github.com/stellar/go/tree/master/services/horizon and Stellar Core at https://github.com/stellar/stellar-core