A Lot is a unit of measurement for transaction size, which is the traded quantity of the base currency of a currency pair.
A standard contract is 100,000 units of the base currency. Therefore, most trades are conducted with leverage because traders may not necessarily have 100,000 euros or whatever currency they are trading to put into each trade.
Micro Lot: equals 1000 unit of the base currency.
Mini Lot: equals 10,000 unit of the base currency.
Standard Lot: equals 100,000 of the base currency.