You can find many types of chain saw sharpeners on the market. They run from the round file, manual operation, to the fancy electric ones that work well for small jobs, to the professional units that you can use to run a chainsaw sharpening business.
The simplest type is a round hand held file that you bring with you to the job site. It is inexpensive and works to get you out of a bind should you ding your chain in the field.
Even professional loggers carry hand held chain sharpeners. They only cost a few dollars, and can save you time and money if you cut wood for a business. You need to pick a file that matches your chain and the teeth pitch. The most common sizes are 1/4" and 3/8" files.
You use the round file along with a hand held guide that fits on your chain to maintain the correct angle as you sharpen your saw. This type is inexpensive and can be purchased for about $15 in many hardware stores.
Moving on up you can buy an electric chainsaw sharpener. These sell for about $75, such as the Garrett Wade 115V 45T01.01 or the Garrett Wade 45T01.02. You can get these with an automatic chain feed.
Another type of chainsaw chain sharpener is a bar mounted guide. This types of sharpener can be taken with you and used in the back of your truck. The guide is secured to the bar of your chainsaw. These units cost about $100.
Finally, you can buy a professional bench mounted chain saw sharpener. This is what your dealer uses. Unless you are going into the business this probably wont pay for itself because you do not want to get a cheap model. The less expensive units do not cut the curved shape of the tooth.
If you do any amount of work with a chainsaw, buying a hand held sharpener to use on a regular basis between professional sharpenings will be the best investment you can make.