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DNA

Deoxyribonucleic acid, oversimplified: Stores the genetic code that determines everything about you from hair color, height, diseases you are susceptible to, and so on.  Known as a Double Helix. They are the building blocks that make us the same and different, all at the same time.  While your life experiences will help mold you into the person you are, it is traits handed down through your DNA that will arguably possibly make you more likely to do certain things, act certain ways and think about certain things that will lead you to your experiences.

We get 50% of our DNA from each parent who are really just carriers from our grandparents.

The 50% you get from each of your parents will not be the exact same 50% your siblings are given unless you are identical twins.  As an example, let’s say your father’s DNA is labeled 1 thru 100.  You might have been given segments 25 thru 74 while your sister received segments 1 thru 13 and 57 thru 93.  Thus you both only received segments 57 thru 74 that were the same.  This could potentially explain why your hair and eye color are brown and your sister is blonde-haired and blue-eyed, you didn’t receive the same DNA that gave you each those features.  You might have gotten the DNA that governs those traits from your father, since he had dark hair and brown eyes, while your sister got her DNA governing those from your mother who has blonde hair and blue eyes.  Meaning the DNA governing hair and eye color did not fall in the segments 57 thru 74 that we received in common.

What are the main DNA and their tests, used in genealogy?

Autosomal DNA

An overall snapshot of your heritage as it was handed down to you from your grandparents through your parents.  It cannot be 100% accurate but can give you a good idea of how distant a relative might be from you.  This is the test you see advertised by Ancestry.com, My heritage and 23 and Me among others.

yDNA

This is DNA that is handed down from father to son directly, but not to daughters, with very little, if any, difference in the yDNA from generation to generation.  Thus women cannot take this test.  Whether you are related to someone through your direct paternal line or not, is definitive with this test, but it cannot tell you how closely you are related.  This test is offered through sites like Family Tree DNA and is part of the 23 and Me test.

mtDNA

DNA that is handed down from mothers to all of her children, this differs from yDNA in that both sexes inherit the mtDNA.  However, although males inherit it, they do not pass it along.  Only females will pass it along.  Both sexes may take this test.  As with yDNA, this test will definitively say you are related through your mother’s line but not how closely.  This test is offered through sites like Family Tree DNA and is part of the 23 and Me test.