Vehicular Ad-hoc Network (VANET), it is a subclass of Mobile Ad-hoc Networks (MANETs). There are no fix infrastructure of network but relies on vehicles to deliver network functionality. Here, vehicles are equipped with an On-Board Unit (OBU) that makes vehicles to send and receive information from other vehicle or workstation in the Network. Security is also critical issue in VANET as attackers can attack in different ways on this life saving vehicular network. Here in this paper we will come across VANET, various classes of attacks in VANET and different Intrusion Detection System (IDS) approaches.