Audit services could be considered a type of assurance service but they differ in that audit services are only designed to test the validity of the financial statements. An audit service is an official and independent examination performed by an auditor to ascertain the validity and reliability of a companys financial and accounting records as well as supporting documents and the systems and processes responsible for recording and summarizing that information with the goal of expressing an opinion regarding their fairness and presentation( hindi na ako makahanap ng ibang words, hehe).
Attestation services, on the other hand, are services in which the practitioner, usually a CPA or an attorney, engages to issue or does issue a written communication that expresses a conclusion about the reliability of a written assertion made by another party. It usually comes in the form of a letter that states whether or not the practitioner believes that the information provided by the party responsible is as accurate as he claims it to be. By attesting to it, the practitioner is saying that he believes the information to be accurate and trustworthy as the responsible party claims which lessens the risks regarding its reliability.