Upfront all transactions must pay for the computation they expect to have. The user (or whatever) decided how much has may be spent. When the transaction runs,

Upfront all transactions must pay for the computation they expect to have. The user (or whatever) decided how much has may be spent. When the transaction runs, gas is consumed. If the transaction finished or there is no more gas, computation stops. Now, transactions have no limit but blocks of transactions have. This is to prevent a single person creating and mining gigantic transaction to stall the network. This limit is dynamic and you can raise it by having the network use more gas until the moving average climbs high enough.