A Mobile Ad-hoc NETwork (MANET) is a self-organized wireless short-lived network consisting of mobile nodes. Therefore, security in MANET is the most important concern for the basic functionality of network. MANETs must have a secure way for transmission and communication and this is a quite challenging and vital issue as there is increasing threats of attack on the mobile networks. Security is the cry of the day. In order to provide secure communication and transmission, the engineers must understand different types of attacks and their effects on the MANETs.