According to the Statements on Standards for Attestation Engagements published by the AICPA, "an agreed-upon procedures engagement is one in which a practitioner is engaged by a client to issue a report of findings based on specific procedures performed on subject matter. The client engages the practitioner to assist specified parties in evaluating subject matter or an assertion as a result of a need or needs of the specified parties." Essentially an agreed-upon procedures engagement is an attestation engagement that is less comprehensive than a review or engagement, but more flexible in nature.
As an example, sometimes a third party will need assurance as to the correctness of the balance of a specific account or group of accounts on the financial statements. Since this assurance does not relate to the financial statements taken as a whole, the time and cost associated with performing a full audit or review on the financials would not be justified by the need to verify only a portion. An agreed-upon procedures engagement may prove an effective alternative for your firms attestation needs.