A block is a bundle of transactions that gets added to the chain all at once. That is really all it is. Each block has a header containing metadata (a timestamp, a reference to the previous block, a summary hash of all the transactions inside it, and whatever other fields the specific chain requires), followed by the transactions themselves. The block is then signed or mined according to the chain’s consensus rules and broadcast to the network, and once other nodes accept it, it is permanent.
The reason blockchains batch transactions into blocks rather than processing them one at a time is efficiency. Broadcasting every single transfer as its own discrete update would flood the network. Grouping a few thousand transactions into one bundle, adding a single proof that the bundle is valid, and broadcasting the bundle is far more bandwidth-efficient. The cost is latency — you have to wait for a new block before your transaction is confirmed.
How often new blocks arrive depends entirely on the chain. Bitcoin targets one block every ten minutes, Ethereum one every twelve seconds, Solana one every 400 milliseconds, Polygon one every two seconds. These are all policy choices and they have different trade-offs. Faster blocks mean better user experience but more orphaned blocks (two nodes producing a block at the same time and only one winning). Slower blocks are more robust but feel sluggish.
How Many Transactions Fit
Each block has a size limit. Bitcoin’s is roughly 1 megabyte of transaction data, which comes out to between 1,500 and 3,000 transactions per block depending on how complex they are. Ethereum uses a gas limit rather than a byte limit — currently 30 million gas per block, which translates to around 100-250 transactions depending on what they do. Solana blocks are bigger and faster and can hold several thousand transactions each.
When demand exceeds what blocks can hold, transactions wait in the mempool (the queue of pending transactions), and users bid up fees to get included faster. This is the mechanism behind every Bitcoin or Ethereum fee spike you have ever seen — a block is full, the backlog is growing, and whoever pays more gets in first.