IBM APM versions

IBM Application Performance Management comes in four editions. There is a good explanation in the knowledge center on differences:
APM offerings 

If you need standard OS and application monitoring you can normally use the basic version which is IBM Monitoring. It also includes the "Realtime transactions agent" which listens for HTTP-traffic. This is something you will use on your webservers to analyze web-traffic and it's quite powerful. Not many other products has this capability in the price-range that IBM Monitoring is in.

The IBM Application Performance Management edition adds in transaction tracking and a really cool dependency analyzer which draw your application topology. It does not work for everything, but I've seen it work very well with WebSphere.

There are a few more agents in this version which is not in the IBM Monitoring package.

IBM Application Diagnostics is for environments with WebSphere and/or WebSpehre Liberty Profile. It provides deep-dive functionality for such environments and it's a great tool to have in place.

IBM Application Performance Management Advanced is IBM Application Diagnostics + IBM Application Performance Management.