A simple explanation is that each time a new block is created ("mined"), the miner puts some or all of the outstanding transactions in the block, which "execute

A simple explanation is that each time a new block is created ("mined"), the miner puts some or all of the outstanding transactions in the block, which "executes" them. The miner is allowed to pick which of those transactions he wants to include using any criteria he wants. A person making a btc transaction can add "transaction fees" to the transaction, effectively making a second transaction to the miner of the block who accepts his transaction. This allows for a simple anti-spam/prioritization feature: The miner sorts outstanding transactions by transaction fees offered, and puts the top n txns into the block. The total volume of transactions that are handled by the system is designed to be large enough to be practical for all and any uses. This means that at the moment if someone isn't embedding files in the block chain or something (BTCfs is a real thing!), transactions without fees will still typically be executed eventually.