A starter is used for a couple of reasons.  The main reason is to increase the amount of yeast cells you pitch into your beer.  By increasing the number of yeast cells you speed up the time it takes for the fermentation to start.  This is something you want to try to do since from the time you put the wort in the fermenter and the time it takes (usually 8-24 hours) to start fermenting, bacteria can also start to grow.  One thing that yeast does before they start to eat the sugar in the wort is multiply.  When you make a starter they are multiplying.  In fact, a starter is nothing more that a very small batch of beer.  You can step up the starter several times if you want.  I usually just do the process one time for regular beers and two to three times for high gravity beers.  The other reason you make a starter is a way to make sure you yeast is alive and healthy.  You usually make the starter a day or two days from the time you will be brewing. 

To make a starter you will need the following:

1. Glass jar that will hold a couple of quarts (I use a gallon apple juice jug)
2. Airlock
3. Stopper (I use a number 8 with the gallon glass jug)
4. Sauce pan
5. DME (dried malt extract 1/2 cup)
6. Yeast
7. Measuring cup
8. Water (2 cups)
9. Sanitizer

First put 2 cups of water into the sauce pan and heat until it starts to boil.  Take off the heat and add the 1/2 cup DME.  Put back on stove and let come back to a boil.  While this is going on sanitize your jug, airlock, and stopper.  After it has boiled for 5 minutes, take off the stove and put in a ice bath to cool.  Once it has cooled below 80 degrees, pour into jug and swirl around to aerate for a couple of minutes.  Yeast need oxygen to work and by boiling you purged most of the oxygen out.  Then put the airlock and stopper in place.  That's it!