Together they create a powerful second-level network, a wholly different vision for how the internet can function.
Every node is an “administrator” of the blockchain, and joins the network voluntarily (in this sense, the network is decentralized). However, each one has an incentive for participating in the network: the chance of winning Bitcoins.
Nodes are said to be “mining” Bitcoin, but the term is something of a misnomer. In fact, each one is competing to win Bitcoins by solving computational puzzles. Bitcoin was the raison d’etre of the blockchain as it was originally conceived. It’s now recognized to be only the first of many potential applications of the technology.
There are an estimated 1600 Bitcoin-like cryptocurrencies(exchangeable value tokens) already available. As well, a range of other potential adaptations of the original blockchain concept are currently active, or in development.