What is Intrusion Detection System

An intrusion detection system (IDS) generally detects unwanted manipulations of computer systems, mainly through the Internet. The manipulations may take the form of attacks by crackers.
An intrusion detection system is used to detect several types of malicious behaviors that can compromise the security and trust of a computer system. This includes network attacks against vulnerable services, data driven attacks on applications, host based attacks such as privilege escalation, unauthorized logins and access to sensitive files, and malware (viruses, trojan horses, and worms).
An IDS is composed of several components: Sensors which generate security events, a Console to monitor events and alerts and control the sensors, and a central Engine that records events logged by the sensors in a database and uses a system of rules to generate alerts from security events received. There are several ways to categorize an IDS depending on the type and location of the sensors and the methodology used by the engine to generate alerts. In many simple IDS implementations all three components are combined in a single device or appliance.

What is Intrusion Prevention System

An intrusion prevention system is a computer security device that monitors network and/or system activities for malicious or unwanted behavior and can react, in real-time, to block or prevent those activities. Network-based IPS, for example, will operate in-line to monitor all network traffic for malicious code or attacks. When an attack is detected, it can drop the offending packets while still allowing all other traffic to pass. Intrusion prevention technology is considered by some to be an extension of intrusion detection (IDS) technology. The term "Intrusion Prevention System" was coined by Andrew Plato who was a technical writer and consultant for *NetworkICE.
Intrusion prevention systems (IPS) were invented in the late 1990s to resolve ambiguities in passive network monitoring by placing detection systems in-line. A considerable improvement upon firewall technologies, IPS can make access control decisions based on application content, rather than IP address or ports as traditional firewalls had done. As IPS systems were originally a literal extension of intrusion detection systems, they continue to be related.
Intrusion prevention systems may also serve secondarily at the host level to deny potentially malicious activity. There are advantages and disadvantages to host-based IPS compared with network-based IPS. In many cases, the technologies are thought to be complementary.
An Intrusion Prevention system must also be a very good Intrusion Detection system to enable a low rate of false positives. Some IPS systems can also prevent yet to be discovered attacks, such as those caused by a Buffer overflow.