Blockchain is a type of distributed ledger technology, which according to Dan Tapscott, "is an incorruptible digital ledger of economic transactions that can be programmed to record not just financial transactions but virtually everything of value.”
Robin Chauhan, on their Medium, offers another interesting interpretation of blockchain:
“Blockchain is a record of transactions, spreading across the internet as more people use cryptocurrencies. Similarly, DNA is a record of genetic transactions and mutations that spread as life expanded across the earth. Both become more complicated over time as our DNA evolves and new blocks are added to the blockchain.
Each blockchain (Bitcoin, Ether, Ripple) is like a distinct species (human, chimpanzee, etc.). A blockchain can also be forked (like with Bitcoin Cash) to create a competing currency in the same way that two distinct species can share common ancestor.
In terms of transactions, they are organized into a series of blocks, which is the blockchain. A new block is created once verified. It means that a new pool of encrypted transactions (possibly including yours!) have been added to the blockchain.
In Proof of Work, miners (imagine gold miners) get rewarded for processing transactions by solving complex mathematical problems, known as hashing. These problems are so difficult that it would take an impractically large amount of computational power to solve them. The miner who solves the problem first gets to add the next block to the blockchain and is rewarded with a certain number of cryptocurrency coins, such as Bitcoin. This process is known as mining and helps to secure the blockchain by making it difficult for any one person or group to control the majority of the computational power. As a result, Proof of Work is considered to be a decentralized and secure way to maintain a blockchain.