Blockchain is a decentralised, distributed and public digital ledger that is used to record transactions across many computers so that the record cannot be altered retroactively without the alteration of all subsequent blocks and the consensus of the network. This allows the participants to verify and audit transactions inexpensively.
Having a contracts between two/more parties have changed the way, after Blockchain. Now all the parties including lawyers and Government bodies can use distributed ledger technology to make this process efficient than ever.
Businesses who set up a private blockchain, will generally set up a permissioned network. This places restrictions on who is allowed to participate in the network, and only in certain transactions. Participants need to obtain an invitation or permission to join.
A public blockchain network is completely open and anyone can join and participate in the network. The network typically has an incentivising mechanism to encourage more participants to join the network. Bitcoin is one of the largest public blockchain networks in production today.