Encumbrance is concept of fund reservation at every stage of your business flow. For example: When you raise a PO the fund will be reserved for the PO account. Now when you receive an invoice for this PO. The PO fund will be unreserved and a reservation entry will be made for the invoice enc account.
Accrual is an accounting practise which says that "Your revenue and expenses should be recognized in the same period". This gives a better/truer picture of how the business is doing and is followed widely.