Photoshop Elements - Filters > Sharpen > Unsharp Mask.
Sharpening is something you may want to do immediately before printing. Always make it the last thing you do before printing - sharpening can upset the way other adjustment tools work.
After you've made several adjustments to a picture you may start to lose some of the contrast around the edges of details. Sharpening helps to bring this back.
Sharpening does not create any detail which has been lost or was never there - it only makes the picture appear sharper.
'Sharpen' and 'sharpen more' are crude tools that can be used for quick results. 'Unsharp mask' gives you much more control. Experiment with the sliders to get the effect you want. A good start may be: Amount = 100%, Radius = 1.0 and Threshhold = 0.
Do not oversharpen - it leads to bright ghost effects.
Use the zoom tool to set the size of the screen image to 100%, otherwise you will not see the true effect of sharpening.
Only sharpen a picture once. If you are not happy with the result go back to the original rather than trying to add more sharpening to an already sharpened image.
Most digital cameras will already have done some sharpening. If yours already does a lot of sharpening then be careful about doing any more. Some cameras allow you to adjust the amount of in-camera sharpening; I prefer to turn it right down and control it myself in Photoshop.