Receiving payments with Bitcoin is almost instant. However, there is an average delay of 10 minutes before the network starts to confirm your transaction by including it in a block and before you can spend the Bitcoins you received. Confirmation means that there is a consensus on the network that the Bitcoins you received were not used to pay someone else and are therefore considered yours. Once your transaction is included in a block, all subsequent blocks will include it, which greatly strengthens the consensus and reduces the risk of transaction reversal. Each user can decide for themselves when a transaction is confirmed, but generally speaking, receiving 6 confirmations is as safe as waiting 6 months after a credit card transaction.
