APM is the newer version of the monitoring product from IBM. Here are some high-level differences.
While ITCAM was available as a paid service in some form earlier, APM is a real Cloud based SaaS offering. You can buy a subscription online and get going quick. For ensuring high uptime, add-ons like synthetic monitoring and predictive analytics are available.
APM can be also installed on-premise as well as hybrid installation.
The installation is pretty simplified; one has to install only one server as compared to ITCAM. Both the above items, means a greatly improved TTV for the customers.
It has a redesigned modern web based UI, fit for the mobile world.
Some of the newer technologies can be monitored (currently!) only via APM , which makes sense being the newer of the two,.