Simply put, an intrusion is someone attempting to break into or misuse your system. How you define someone and break into or misuse is up to you. An intrusion detection system , or IDS for short, attempts to detect an intruder breaking into your system or a legitimate user misusing system resources. The IDS will run constantly on your system, working away in the background, and only notifying you when it detects something it considers suspicious or illegal.
Most people perceive the outside world to be the largest threat to their security.
Think about it - an insider knows the layout of your system, where the valuable data is and what security precautions are in place. So despite the fact that most security measures are put in place to protect the inside from a malevolent outside world, most intrusion attempts actually occur from within an organization. An effective intrusion detection system detects both types of attacks.