"The combination of crash reports provided by the iPhone and System Activity Monitor helped me determine that I had an issue with contact syncing that caused my iPhone to drain with alarming rapidity." - http://www.macworld.com/article/1163200/troubleshoot_iphone4s_battery.html

"This App is similar to the Mac OS Activity Monitor in that it shows you in real time which apps or processes are running and more importantly which ones are running with high amounts of processor time. If you're not playing a game or doing something else that would task the processor then your device should be running with the majority of the processor count in "idle." If it's not, you can quickly identify which App or process is causing a problem.

I highly recommend System Activity Monitor as a troubleshooting tool!"
- http://www.bestappsite.com/tag/system-activity-monitor/

"System Activity Monitor is simple and it works quite well, without any frills or flashy graphics." - http://www.appsafari.com/utilities/17457/activity-monitor-iphone-apps-useful-for-solving-iphone-4s-battery-life-issues/

"5 STARS!!!
This the first rating I have ever given because this app gave me my IPAD back. it was crashing all the time until I purchased this app. Give it a few seconds to work it's magic. The memory will free up and the CPU usage will drop from 98% to 2%."

NOTE: Turn on Backward Compatibility (in the Settings app) to get the previous pre-iOS 5.x behavior.

System Activity Monitor (TM). It's like the Activity Monitor on your Mac but for your iPhone.

You can view:
- Runtime (RAM) memory usage
- Storage (Disk) usage
- All running processes
- CPU usage
- WiFi IP address
- Device information

Memory types:

Free memory
This memory is not being used currently

Wired memory
This information can't be moved to disk, so it must stay in RAM. The amount depends on the applications you are using.

Active memory
This information is currently in RAM and has recently been used.

Inactive memory
This information has not recently been used but will remain in RAM until another application needs more memory but no free memory is available. If called upon by a process, this is quickly changed to Active memory; if it has been swapped to the hard disk, it will be moved back to RAM and marked as Active.

Page ins / Page outs
"Page ins/outs" refers to the amount of information moved between RAM and the hard disk. This number is cumulative amount of data that iPhone OS has moved between RAM and disk space.