What is the difference between tippet and plain old mono or fluorocarbon? Can't I just use fluorocarbon and save some cash?
Tippet comes in small rolls such as this:
Whereas you can buy a roll of fluorocarbon cheaper:
You CAN use the fluorocarbon for tippet. In fact many high quality tippets ARE made of fluorocarbon.
What you get by buying the smaller roll of tippet is portability and the ability to conveniently carry several different sizes.
When fly fishing you will need to adjust your tippet according to the size/type of fly you are using and to how leader shy the fish are.
A heavier/bulkier fly will require heavier tippet just to permit a well controlled delivery. If you are throwing very small dry flies you might need 7x or smaller tippet to reduce the line's effect on the fly as it floats in the water or to minimize the chance of spooking a trout.
In a given outing I regularly change my tippet (at the end of my leader) 3-4 times. Sometimes I'll increase the leader length by adding a bit of 4x, and then tying on 5x, and then some 6x - the idea is to drop down sizes as you go so that each subsequent section requires less energy to turn over - giving a smoother delivery.
Carrying large rolls of fluorocarbon isn't practical on the stream or bay. Also you may not be able to find large rolls of very fine tippet.
Yes. You can use whatever you want for the tippet. There is no fly fishing police that enforce tippet rules. Not like the police who enforce the fishing license rules, the barb-less hooks rules, or the targeting fish out of season rules.
Granted, your choices of tippet material may produce differing results but nothing works in every situation. Give it try and see what happens. If you're not happy with the results, try something else.
The other two answers are correct. You can use plain old nylon or fluorocarbon monofilament lines and save cash (with the pitfalls mentioned by the others).
Keep in mind, however, that specialised tippet brands sometimes use better materials or do fancy things to their lines like coating them with UV or abrasion resistant materials, so you may want to go for them if you want good quality. It is possible to buy large bulky spools of specialised fly fishing tippet (like Stroft, incidentally one of the best in the market) from some brands, so you still get to save money.