how do I post a screenshot from mac OS X lion on askdifferent?

How do I take a screenshot? On the other Mac I was using, grab was installed, but paying for screenshots seems excessive.

System Software Overview:

  System Version:   Mac OS X 10.7.5 (11G63)
  Kernel Version:   Darwin 11.4.2
  Boot Volume:  Untitled
  Boot Mode:    Normal
  Computer Name:    a MacBook
  User Name:    thufir (thufir)
  Secure Virtual Memory:    Enabled
  64-bit Kernel and Extensions: No
  Time since boot:  53 minutes

More specifically, how do I post that screenshot here on askdifferent?

I see that you can take a screenshot...but...where is it saved?