How to check the system resource usage?

Today is my second day of usage a Macintosh computer, so far so good. But it is not as convenient as i first imagine. Yesterday I installed a freeware call "Free Memory", it is an application which shows the amount of free memory on the menu bar. Running the system for quite a while, I find that the amount of free memory becomes an unbelievable low level, only about 20MB free! I have bought 4G RAM for my MacBook Air, how come it can drop to such a low level?!

To check the system resource usage, do the following easy steps:

1. Open Finder
2. Select Applications from the left side
3. Open Utilities
4. Start "Activity Monitor.app"

You will get a window like that:


Why my memory was in such a low state? Because i try to use VirtualBox to run a window inside. That is a memory hog. But i don't really know how is Mac doing on the memory management.

In Windows, I can Ctrl+Alt+Del to open a Task Manager, which show a nice graph of CPU usage, can Mac have something like that showing on the screen? A CPU Widget?