A Forex Lot is the amount of currency you buy or sell. Say for example you wish to buy 100,000 USD, 100,000 is basically your trade size.
Forex Lot is basically representation of Trade Size in a different format. A Standard Lot would represent 100,000 of any currency, whereas a Mini Lot represents 10,000 and a Micro Lot represents 1,000.
So essentially 10 Mini Lots make 1 Standard Lot and 10 Micro Lots make 1 Mini Lot.
100,000 of a currency represents one Standard Lot or ten Mini Lots or one hundred Micro Lots in Forex market.
100 Micro Lot = 10 Mini Lot = 1 Standard Lot
When you place orders online you should know what account you have and how your broker lets you trade. For example with MBT, you can have an Standard Forex Account and still place trades in Mini Lots. On the other hand with Dukascopy you can only trade Standard Lots.
You may also want to read about Different Type of Forex Accounts.