Depth of Field or DOF is how much of the photo is in focus. With shallow depth of field only the parts that are closest to the camera (foreground) are in focus and the rest is out of focus. Deep depth of field means that the background and foreground is in focus.
Deep DOF is great for landscapes but shallow DOF is great for portraits or anything where you want to bring attention to the foreground and gently blur out the background.
Here's a great tutorial on how to achieve a similar effect of shallow DOF using Photoshop Elements. Of course a software program can't take the place of the lens but it gives a good result. The only problem so far that I can see is that this feature is only available in PSE10 and not PSE9.
You can use a mask and a blur effect to achieve the same result.
Here is also another way to create DOF in PSE except there they use gradients.