A block is a container data structure. In the Bitcoin world, a block contains more than 500 transactions on average.
So, a block is composed of a header and a long list of transactions.
If you are a miner, your job is to gather transactions from the transaction pool in to a “candidate block”, and to try and add this candidate block to the blockchain. Blockchain is a distributed database existing on multiple computers at the same time. It is constantly growing as new sets of recordings, or ‘blocks’, are added to it. Each block contains a timestamp and a link to the previous block, so they actually form a chain. The database is not managed by any particular body; instead, everyone in the network gets a copy of the whole database. This ledger is visible on every node participating in the network. Old blocks are preserved forever and new blocks are added to the ledger irreversibly, making it impossible to manipulate by faking documents, transactions and other information
Batches of these valid transactions are the primary contents held in a block. Blocks can also be programmed to include other data like timestamps, block height, etc., pertaining to different blockchains requirements.