The Lasso tool
There are three types of Lasso tools (shortcut L) available to help you select part of an image. There's the Lasso tool and Polygonal Lasso tool which you can draw freehand with as well as create straight-edged selections. Then there's the Magnetic Lasso tool which is a handy tool to make selections with when you have a shape/object sat against a high contrast background.

To select an object with the Lasso tool just click your mouse near the object you want to select in your shot and drag the cursor around it until all of the object is selected, as you would when circling something with a pen on a piece of paper. When you return to your starting point your selection will turn into a line of 'running ants'. It's an easy tool to use as you simply drag your cursor around but it can be tricky trying to make a precise selection if you've not used it before. However, it does make it easy to select awkward shapes once you've had a bit of practice drawing free-hand. The various selection commands that were discussed above also mean you can go back and adjust your selection, adding and removing to/from it until you're Happy.