... enter terminal in Spotlight search field and click Terminal (see picture).
A command entry window shows up. Mac OS X is built on a Unix platform. Terminal is a command entry application where you can enter all the Unix like commands to make some tweaks to the Mac OS you otherwise couldn't by only clicking through the usual Settings and Preferences.
Let's say you want to save screen shot images in Pictures/Screenshots folder, you need to enter following line into Terminal window.