There are many filters (under Filter on the menu bar) to produce all sorts of effects. Many of them are next to useless. The best way to find out what they do is to experiment with them.
The two you are most likely to use are 'sharpen' and 'blur'. We learned about sharpening in Session 3.
Blurring works in much the same way as sharpening. However there are several different types of blurring (Gaussian is probably the most useful).
Blurring can be very useful to tone down a distracting background or to smooth out those wrinkles.
Under 'Enhance' can be found several 'auto' tools - auto levels, auto color, auto contrast.
They may be fine for a quick fix but don't rely on them if you want the best quality.
With this tool you can make most of the adjustments you could have done with levels, colour variations, brightness, contrast, etc., but with greater control.
You use it by clicking on the curve (which starts as a straight line) and dragging it up, down or sideways. It's difficult to describe - the best thing is to try it yourself.
You can make overall changes using the RGB channel, or you can make changes to individual colour channels.
Tip: Unless you want psychedelic results, make only small changes.
Tip: Experiment with shallow 'S' curves. One way they increase contrast, the other way they bring out detail in the shadows and highlights.