A yeast starter is the best way to build up colonies of yeast to ensure proper fermentation. While it is possible to ferment a standard batch with one packet/vial/smack pack of yeast, it is recommended that a starter be made from it anyway to ensure that there are many viable yeast cells and that the yeast is active when pitching to avoid long lag time.
 Reasons for using a starter
1- More viable yeast cells
2- Shorter lag time
3- Healthier fermentation
4- Reduces the chances of underpitching
 How to make a yeast starter
1- Make a wort of some kind. The simplest way to do this is to boil up some LDME.
2- Some will say it is best to aim for a gravity similar to the one that is being aimed at for the batch (to acclimatise the yeast). [Others] believe that a wort with a SG of 1.040 is best for yeast production.
3- Pour your 'wort' into a sanitised container- note that sanitation is crucial when it comes to yeast starters.
4- Ensure that the starter is at the right temperature- that is, the temperature you intend to ferment at. If it is at a higher temperature, the yeast adjust their metabolic rates during succesive cell division and become "aclimatised" to that temp.
5- Pitch the yeast into the starter.
6- Seal the container, but make sure that the CO2 can leave- some attach an airlock to a rubber bung, others use a rubber balloon.
7- Some step up their starters into larger volumes over several days if a large volume of wort is being fermented.
8- When the yeast is active, pitch it into your brew.