How would you explain blockchain technology to a layman? How is it decentralized?

A blockchain is a list of transactions that are grouped together into blocks, with each block being validated.

For the earliest blockchain, Bitcoin, transactions are transfers from one account to another. For later blockchains different types of transactions were created. For example Steem transactions include upvotes and posts as well as transfers.

For a decentralised blockchain there is no central entity responsible for validation of blocks. Each node on the blockchain network has a copy of the the list of transactions and when new transfers are made, new blocks of transactions are filled and then validated by the nodes.

Validation is necessary to ensure there is consensus between all nodes in the network as to the correct list of transactions and to prevent single nodes from creating fake transactions.

The process of validation varies depending on the type of blockchain.

Proof of Work (PoW) blockchains such as Bitcoin use a computing power competition for each block validation together with a rule that the longest chain takes precedence. A node would need more than 50% of the overall network computing power to make the longest chain themselves and validate fake transactions.

Proof of Stake (PoS) blockchains choose a random node for each validation, with the chance of being chosen dependent on the amount of Stake (i.e. the amount of the blockchain currency) held by the node. Nodes with more Stake will validate more blocks.