Bitcoin is a form of electronic money. It is different to the traditional fiat money system (Dollar, Euro etc.) in three key ways:
The blockchain is the core of Bitcoin as it is the summary of all the transactions in the Bitcoin Network. As the name implies it is a chain of blocks, or sets of transactions in chronological order.
Each block gets added to the blockchain after a miner finds a hash that is accurate for that particular set of transactions. That hash along with the hash of the previous block is then distributed throughout the network and by consensus added to the most recent end of the block chain.
The Bitcoin network is a peer to peer network (P2P) or a decentralised network with no central point of failure or command. All the nodes, or computers, that participate in the network are equal – there is no hierarchy – or no topology as the network is flat.
The Bitcoin network is a collection of nodes running the Bitcoin P2P protocol with other protocols such as stratum, used for lightweight mobile wallets (where the full block chain is not downloaded) or for mining.