The most basic need or application of a blockchain is to carry out transactions or exchange of information through a secure network. But the way people use blockchain and distributed ledger technology or network vary from case to case. For instance, if we talk about Bitcoin, which is how blockchain got introduced in the mainstream. Bitcoin is a digital cryptocurrency which gets transacted through the blockchain and DLT technologies. This type of blockchain network is a public network because people from all over the world can become a node, verify other node and trade bitcoins.
On the other hand, let us suppose that a bank is using a private blockchain network. It will be a restricted network where only the authorized members of the bank can access confidential information. Thus, no one out of this closed network can gain access to bank data. A private network will have limited and authorized nodes monitored by a network administrator. The information transmitted through such a private blockchain network stays within the network. Any new node that wishes to get added in a private network needs permission from the network admin. The bank gets to decide the scale of their private blockchain for all the branches of a city or all their branches in a country. Just like these examples, there are different ways in which the blockchain network is set up depending on the use and requirements.