DNA tests that are used in genealogy are using very small snippets taken out of a very long DNA molecule. Current genealogy DNA tests do NOT identify your ethnic background, or any hereditary problems like defects of susceptibility to any disease.   Thus the term "Junk DNA" describes where genealogy information is found.
There are two kinds of DNA Tests:
Y DNA - tests males for the Y chromosomes that are passed down from father to son, to son, etc.
Y tests started with just a few  markers being tested and now can test for dozens of  markers that have been passes down.

The Y markers are subject to occasional changes thus a son may not have exactly the same test result as the great-grandfather.   Because of these changes, results can be a predictor of how many generations you will go back to find a common ancestor.
Mitochondrial DNA -  test males and females for DNA passed down from mother to child.
The Mitochondrial DNA rarely has changes from generation to generation thus cannot be reliable to say how close similar results are related. 

The higher number of markers tested is like using a larger telescope to view stars.  My first test a long time ago confirmed my placement in the family tree and identified a distant cousin who was also in the same tree.    In the future, as more people get DNA tests, the connections learned may be enormous.
Privacy will always be a concern and your privacy requests must be adhered to.  
You may want to never divulge your results but want to use it to compare to others who allow their results to be known.
You may want to release your results under an alias so that others can see your results and post comments for you to see.
You may have no concerns and freely post your DNA results along with email address, home address, and even telephone number.
According to information online at http://dna.ancestry.com you may be able to confirm or deny branches in your tree.   I may find that the two Ellis lines in our tree are or are not related.   That could save me a lot of time.  You could identify other researchers who could have a vast storehouse of data on part of your tree.  
Researchers have already identified DNA patterns of the early ancestors and you may be 99.99% pure Japanese but have Attila the Hun's DNA pattern.  Don't be concerned to much as this snippet of information may make your tree very easy to trace.
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I do not have all the answers but I'll gladly look for information.   I will gladly research your questions posted here.  Please post your experiences here to so that others may learn from them too.   ( Allen Wheatley http://teafor2.com      DNA@teafor2.com )