Great article and examples on Pioneer Woman Photographer showing the use of Depth Of Field. That's using the A - Aperture setting on your camera.
The Pioneer Woman Photographer blog site is in the top 5 nominations for Photography Weblogs at the Weblogs Awards 2009
Top photos shot using F-Stop 1.4 and photo above using 11.
1.4 gives a very narrow depth of field (area of photo that is in focus) and 11 has greater depth of field and so more of photo is in focus.
Have a read and if not sure about anything let me know and I'll talk you through it.
ps the type of lens you use can impact this too. That's for another day.