The Different meanings of bitcoin

Bitcoin is an overloaded word, as it can mean several things. Many recognizes it as only digital currency. But there are more to it. to fully comprehend the technology behind bitcoin. There is a need to learn it's different meanings:

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The bitcoin protocol.

The protocol is the specification of how to construct the distributed database (the blockchain), how to parse it, how transactions should be assembled, what constitutes a valid transaction, and so on.read more (http://www.michaelnielsen.org/ddi/how-the-bitcoin-protocol-actually-works/)

The bitcoin network.

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This is the peer-to-peer network to which nodes connect. Nodes in this peer-to-peer network exchange messages containing new blocks being added to the blockchain and new transactions being published. 

The currency.

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A bitcoin, usually spelled with lower case “b”, is a unit of the native currency of the Bitcoin network. There will be a total of roughly 21 million bitcoins issued. Although bitcoin is the main unit of account, each bitcoin is divisible to 100,000,000 pieces, called satoshis 6. This is actually an open source implementation. And also the original open source project, written in C++, implementing the protocol. The project was re-branded to Bitcoin Core, in part to avoid confusion between the different meanings of Bitcoin. Both the source code and complied binaries can be freely downloaded from (bitcoin.org/en/download.)

Bitcoin Core is a single computer program but it includes two different services: Bitcoin Core Wallet, also known as bitcoin-qt, is the default implementation for a wallet. The wallet is a full node wallet as it requires a full node to run. Bitcoin Core Wallet presents a GUI to the user using the qt framework, hence the name bitcoin-qt. Bitcoin Core Server, also known as bitcoind, implements a network node. It can be run in headless mode, i.e. without a graphical user interface, as a daemon, hence the name bitcoind.
Bitcoin Core Server is used to connect to the Bitcoin network, interchange messages with it, interpret the blockchain, handle new transactions in the network, and so on.

Use of bitcoin core?
Bitcoin Core is programmed to decide which block chain contains valid transactions. The users of Bitcoin Core only accept transactions for that block chain, making it the Bitcoin block chain that everyone else wants to use.Also Bitcoin Core ensures every block and transaction it accepts is valid, increasing not only your security but also helping prevent miners and banks from taking control of Bitcoin.