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An IDS (Intrusion Detection System) may only detect and warn you of a violation of your privacy. Although most block major attacks, some probes or other attacks may just be noted and allowed through. An example of an IDS is Black Ice.
A good firewall will block almost all attacks unless specified otherwise or designed otherwise. The only problem is, the firewall might not warn you of the attacks and may just block them. An example of a firewall is ZoneAlarm.
It may be a good idea to have both an IDS and a Firewall, because the IDS will warn you and then the firewall will block the attack. Some Firewall/IDS's are combined into one internet security program, for example Norton Internet Security. This is a very well designed combination of both a firewall and IDS.
Also read About DSL for lots more information