Y goes from father to son only. In other words you get your Y from your father, his father, his father and so on.

Mitochondrial goes from mother both sons and daughters. In other words you get your Mitochondrial from your mother, her mother, her mother.

Autosomal which you get 50-50 from both parents.

Y & Mitochondrial are useful in family history as they go back in a straight line virtually unchanged.
They are helpful in matching you with other family trees. Now no way do they represent all of your ancestry. If you had both done right off the bat they would exclude your paternal grandmother and maternal grandfather. Your ancestry pyramids as you go back. If you get back to your 6xgreat grandparents, your are directly descended from 510 individuals. Of those 510 you get your Y from 8 and your Mitochondrial from 8 leaving out 494 people.

You might say "why bother." If you succeed in matching yourself with other family trees you probably will discover some of the left out people.
It will only help you find your ancestors if you use it as a tool in your family research.

One company that only uses Y & Mitochondrial advertises that they will help you discover your "deep ancestral roots." They will assign you to a Haplogroup based on your DNA and show you the origin of your nomadic ancestors going back thousands of years. However, it will only be in 2 lines and you come from a myriad of family lines.

The best company for Y & Mitochondrial is

http://www.familytreedna.com/Default.aspx?c=1

When you get into their website if you get a "We’re sorry" message, just click on "Home" above and you will have the website. If you click on "feedback" there is a way you can email and them and ask questions.

There is one company that has over 900 batches of 100-150 samples of Autosomal DNA throughout the world. They will take your Autosomal and match you with those samples and give you your top matches in descending order. They won’t tell you that you are 1/2 of something, 1/4 of something else and 1/4 of another something else.
That company is

http://www.DNAtribes.com

When you go into their site if you go under "feedback" there is a way you can email them and ask questions.

Autosomal really represents your total genetic pattern. Your appearance such as pigmentation. bone structure, height comes from Autosomal. Now I had found a website that stated you don’t get your Autosomal 50-50 from both parents so I asked a question about that on the Biology board as Genealogy is not the same thing as Genetics.

This is the reply I got:

"Actually, you do inherit your autosomal DNA 50-50 from each parent. That site is wrong.

"Your mother’s egg contributes one set of 22 autosomes and one sex chromosome (an X). Your father’s sperm contributes the other set of 22 autosomes, and another sex chromosome (either an X or a Y).

"Where the 50-50 part breaks down is when you pass on your DNA to your children. Each of your children will get half their DNA from you, but they won’t necessarily get an equal mix of what you inherited from your parents. They could inherit a more from your mother, through you, and less from your father, or vice versa.

"Another way to look at it: you inherited 50% of your autosomal DNA from each parent, but you didn’t necessarily inherit exactly 25% from each grandparent. Your maternal grandparents contributed exactly 50% in total, but it could be biased in favor of either your maternal grandmother or your maternal grandfather. Same goes for your paternal grandparents.

"When you take one of those DNA tests, which markers they find depend on which ones you inherited through your mother and father, and that will be different between you and your siblings (assuming no identical twins). If one of your grandparents is Dutch, and another is Polish, you might happen to inherit more markers that the test considers NW Eur., and your sister might happen to inherit more that are considered E Eur. In most cases, that will just be a random chance. Also, just because you have more markers that the test considers NW Eur, doesn’t necessarily mean you really are "more" NW Eur. in any real sense.

As to your last question, I’m pretty sure that most of the variation in what DNA you inherit vs. what your siblings inherit is quite random. I don’t think an ancestor from 1700 could somehow dominate the DNA markers in his/her descendents overall. Certain single markers could do that, if they conferred some advantage, but most will be essentially random.
Source(s):
PhD in molecular biology"

I also asked DNATribes if my sister with whom I share both parents had the same Autosomal test would her results be the same and they replied:

Two siblings will each obtain unique results. Family members do typically share some regional or ethnic genetic affiliations, but in some cases matches can vary substantially between siblings."

Here are a couple of more websites that explain DNA
http://www.smgf.org/pages/how_it_works.jspx

http://www.pa.msu.edu/~sciencet/ask_st/060293.html

If you wish to have your Y & Mitochondrial done, the best is FamilyTreeDNA. They do do Autosomal testing but don’t use them for Autosomal. They will not give you an analysis. I think at one time they did.