Micro transactions are a classification of payments that fall below a certain threshold. The threshold for identifying what falls into the Micro payments category is typically one of the two:
1) a transaction that is valued less than the fees to execute that transaction. example; sending a $10 bank wire from the US to Korea will cost no less than $25.
2) a transaction that is valued below 0.001 of a currency (denominations below 1/10 of a penny).