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Since computation is expensive (mind that it has to be done by every full node in the network), excessive consumption of gas needs to be discouraged. Therefore, each unit of gas must be paid for (in Ether) by the sender of the transaction that triggered the computation.
For example, something as simple as transferring DAO tokens from one address to another or casting a vote can reach fees between $0.05 - $0.1 to cover the computation gas.
Transaction gas limit
It is often not easy and in general even impossible to know in advance how much gas a transaction will need eventually. Therefore, transactions have a gas limit field to specify the maximum amount of gas the sender is willing to buy. If the gas used exceeds this limit during execution, processing is stopped.
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