The emergence of the internet has taken the communication system to a new dimension. Governments, military, financial institutions, organizations, universities, individuals and other businesses gather, process and store an enormous amount of confidential information and data on computers, and using the internet the transmission takes place among these computers. The thriving features of the internet also call for implementing robust security measures to protect sensitive information and the whole network. Cybercriminals aim at getting unauthorized or secure information, spying, disabling networks and stealing confidential information for their financial purposes, and misusing data in illegal activity which is termed as cyber-attack. Such attacks have been growing in quantity and complexity over the past few years. It is essential to understand those attacks both before and after they happen to identify vulnerabilities in networks and provide better security to our systems. This paper provides a comprehensive study of these attacks in order to create awareness about the various types of attacks and their mode of action so that appropriate preventative measures can be taken against such attacks.