Intrusion Detection Systems (IDS) and Intrusion Protection Systems (IPS) are a rather broad topic. As such, my answer here is far from comprehensive.
The types of IDS include network and host based.
Network based IDS, such as SNORT, analyze and log network traffic based on a set of rules. These rules would match potential vulnerabilities, thus potentially providing advance warning of attempted intrusion and forensic data after the fact.
Host based IDS include software such as AIDE, which compare hashes of the files on the filesystem on a recurring basis. This would allow someone to monitor changes on the system and identify unauthorized changes.
Central logging could arguably be part of your host based IDS solutions. Central logging enables control and auditing of your logs in a central place. Additionally, keeping logs in a central place minimizes exposure and allows an additional audit trail, in case a system is compromised and the logs are no longer trusted.
Packet filtering (firewalling) is a security mechanism for controlling traffic to and from your network. Firewalls are not IDS.
A well run IT infrastructure includes many of these technologies and many professionals would not consider them optional.