What do these different terms for system memory in OS X mean?
Free memory

This is RAM that's not being used.

Wired memory

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

Active memory

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

Inactive memory

This information in memory is not actively being used, but was recently used.

Used

This is the total amount of memory used.

VM size

This is the total amount of Virtual Memory for all processes on your Mac. 

Page ins / Page outs

This refers to the amount of information moved between RAM and the hard disk. This number is a cumulative amount of data that Mac OS X has moved between RAM and disk space.

Swap used

This is the amount of information copied to the swap file on your hard drive.

See official Source for more details.



The high memory usage has occurred ever since Mac OS X Lion. I don't really now why this is. But generally it's good if your RAM is being used.

If you clear your ram too often, programs start slower because they have to be loaded into the RAM again.