Confirmation
To protect against double spending, a transaction should not be considered as confirmed until a certain number of blocks in the block chain confirm, or verify that the transaction.
The classic bitcoin client will show a transaction as "n/unconfirmed" until 6 blocks confirm the transaction. Merchants and exchanges who accept bitcoins as payment can set their own threshold. When there's loss due to double spending is nominal, payment confirms instantly as soon as it is seen on the network. The eWallet provider, MyBitcoin confirms after one block. Most exchanges and other merchants who bear the risk from double spending require 6 blocks, with the exception of CoinCard which considers payment confirmed after just two blocks.
Coins generated aren't confirmed for 100 blocks, and the classic bitcoin client won't show the generated coins as confirmed until the 120 block.
