You can pay for transactions using Ether. Ether serves two purposes. First, it prevents bad actors from congesting the network with unnecessary transactions. Second, it acts as an incentive for users to contribute resources and validate transactions (mining). Each transaction in Ethereum constitutes a series of operations to occur on the network (i.e. a transfer of Ether from one account to another or a complex state-changing operation in a smart contract). Each of these operations have a cost, which is measured in gas, the fee-measure in Ethereum. Gas fees are are paid in Ether, and are often measured in a smaller denomination called gwei. [1 ether = 1,000,000,000 gwei (10^9)]