Bitcoin Core
Bitcoin is an open source project, therefore everybody can contribute to the code. There are thousands of developers working to improve the network. The first implementation, created by the anonymous founder Satoshi Nakamoto, has been heavily modified. This implementation is known as bitcoin core, to differentiate from other compatible implementations.
Even though Bitcoin Core includes a reference implementation of a wallet, this is not intended to be used as a production wallet for users or for applications. Application developers should using modern standards such as BIP-39 and BIP-32. BIP stands for Bitcoin Improvement Proposal, it is a design document for introducing features or information to bitcoin.
From now on, this will be a highly technical chapter on how to set up a development enviroment for writing bitcoin applications. If you are not interested in this topic, you can skip to the next chapter.