Bitcoin is most known for being the first cryptocurrency and blockchain officially released in 2009. Bitcoin as defined by its creator Satoshi Nakamoto: “a purely peer-to-peer version of electronic cash would allow online payments to be sent directly from one party to another without going through a financial institution… We propose a solution to the double-spending problem using a peer-to-peer network.” https://bitcoin.org/bitcoin.pdf
Computational power is needed to solve complex algorithms called hash algorithms to provide trustless consensus for proof of work blockchains. The time between block formation (Block Time) depends on the computer power (hash power) in the network that is used to mine transaction blocks: to compensate for this variable, a blockchain adjusts the “difficulty” of solving its hash algorithm to maintain its average block time once a specified number of blocks are mined. For instance, Bitcoin's network adjusts its difficulty every 2016 blocks.
A blockchain fork that results from a change of rules in an updated protocol. Updated nodes will no longer maintain the outdated version of the blockchain; however, nodes that do not update their programming may still maintain the original blockchain.