On Ethereum, the blocks have a maximum capacity of 8,000,000 gas units. Each operation has a cost established in gas units. For example, a classic transaction from one account to another costs 21,000 gas units. Transactions using complex functionality in the smart-contracts can cost much more in gas units. Each gas unit has a price denominated in Gwei (1 Gwei = 0.000000001 ETH) defined by market demand. To know the price of the gas unit at any time, we recommend the Chrome Ethereum Gas Price extension.