Cloud computing consists of technologies like distributed computing, grid computing, virtualization, utility computing, network computing, and the web. There are several threats in the cloud, such as man-in-the-middle attacks, port scans, DoS/DDoS attacks, IP spoofing, and phishing. One of the most serious threats today is denial-of-service attacks, because they affect the availability of critical resources. The primary goal of this paper is to analyze the intrusion detection systems (IDSs) that can identify intrusion attempts in distributed systems as well as virtualized environments. This paper discusses various types of systems such as anomaly, signature, hybrid, hierarchical, and collaborative IDS. The systems are studied and the various methodologies, tools, datasets, operating layers, and accuracies of the IDS are compared and contrasted.