What is a yeast starter?

I'm still kind of a newbie at brewing and I see a lot of stuff on the internet/books, etc. about using yeast starters. Just wondering what they are and is it really necessary to use them rather than just using a packet from Wyeast or any other yeast like that? Currently I'm doing the partial mash method, would I use a yeast starter for that or are they typically for all-grain? Thanks for any help.

Ready to go yeast packets are good for a certain amount of wort at a certain gravity for a certain amount of time.  If any of these are outside the parameters of the yeast package, you need to build up the number of yeasty boys to make the wort into beer.  As I like to say, you make wort and yeast make beer.  You don't give them tools to do the job and you get crappy beer.

If I give you a 2 quart saucepan and a book of matches and tell you to make me 5 gallons of wort, you are gonna have a heck of a time making anything decent.  Yeast are like that.  Throw a packet of 100 billion cells where 300 billion are needed to make beer and you will get crappy beer.  When you need more than the packet has, you make a small, clean and sanitary wort and add the yeast to that to build up to a healthy team that is right for the wort you are going to try to drown them in.

You can absolutely just use the Wyeast packet or a vial from White Labs. It will work. I did it that way for a couple of years.

However, providing the proper amount of yeast when you pitch means a faster, healthier start to fermentation which is much better than a slow start and should produce a better beer.

Do you have to use a yeast starter? No. Is it better to use one? Yes.

John Palmer provides good information on how to go about doing a yeast starter. This is from the online edition of his book How To Brew. http://www.howtobrew.com/section1/chapter6-5.html

Read this:

http://www.mrmalty.com/starter_faq.php