A Mobile Ad-Hoc Network (MANET) is a collection of mobile nodes (stations) communicating in a multi hop way without any fixed infrastructure such as access points or base stations. MANET has not well specified defense mechanism, so malicious attacker can easily access this kind of network. In this paper we investigate different types of attacks which are happened at the different layers of MANET after that we discuss some available detection techniques for these attacks. To our best knowledge this is the first paper that studies all these attacks corresponding to different layers of MANET with some available detection techniques.