ACID is an acronym for a set of characteristics that an RDBMS transaction should have. For example, consider a transaction that is the transfer of money from one bank account to another. In this transaction, there is one withdrawal and one deposit.

• Atomicity:
  A transaction should happen completely, or not at all. You do not want the withdrawal of funds without the subsequent deposit of funds. The bank, on the other hand, does not want the deposit of funds without the corresponding withdrawal.
• Consistency:
  A transaction should take the database from one consistent state to another one. The transfer should not be permitted if one of the balances starts or ends in an absurd condition, such as having a negative balance.
• Isolation:
  Changes internal to the transaction should be contained so as not to affect other running transactions. For example, checking the combined account balances just after the withdrawal and just before the deposit should not be allowed, since that would violate the atomicity of the transaction.
• Durability:
  Once the transaction has been performed, it should be permanent. 'Nuff said.