What is the difference between standard accrual and encumbrance accrual?

Dears,

We have recently upgraded to R12.

We are doing the SLA setup.

We would like to have a general idea on the difference between standard accrual and encumbrance accrual.

On 11i we have the setup of the encumbrance on PO's but we need to make sure if this what we need or we just need the standard accrual so we can continue all setups.

Thanks