How firewalls differ from IDS ( Intrusion Detection System ) ? What are the differences between them ?
Taken from Wikipedia on firewall:
In computing, a firewall is a network security system that monitors and controls the incoming and outgoing network traffic based on predetermined security rules
Taken from Wikipedia on IDS:
An intrusion detection system (IDS) is a device or software application that monitors a network or systems for malicious activity or policy violations.
About the difference between both of them:
The main difference is that firewall preforms actual actions such as blocking and filtering while and IDS just detects and alert a system administrator. Here is a small article talking about the differences for further reading.