Disclaimer: I am not a scientist. If you would like to add something here, PM me to discuss it.
This sticky is an effort to combat repetition in the DNA thread.
There are 3 basic types of DNA tests on the market.
Y DNA: the first type of test to be commercially available. Y-DNA is passed from father to son so it is a good tracker of your paternal surname. Depending on how many markers you test (12 was available but now considered too small, 37, 125 and upwards) you will get matches but it’s not easy to determine how far back the match is. Obviously, only men can take this test.
M DNA: tests mitochondrial DNA, passed from mother to daughters (and sons). It has limited value for genealogy at present because so often we lose a woman's name in records.
Autosomal DNA provides matches from either side of your family and out to about 6th cousins. This is the one that most companies are offering. The downside is that you can’t tell on the face of it which side a match is related to.
In all three cases, a traditional family tree based on solid paper research is essential to help you determine where your matches come from.
Many companies are providing these tests. At the time of writing (April ), Ancestry has the largest database (4 million people, mostly in America), Family Tree DNA has about 2 million, but probably has the most Irish and British people. Other companies include 23andMe, MyHeritage and Living DNA, the last two still have quite small databases. This is not an exhaustive list. Several companies allow free transfers to other sites. So, for example, it is possible to test with Ancestry and then do a free transfer to FTDNA and MyHeritage. Pricing varies but is usually around €100. Watch the various companies social media: they all do deals from time to time for Christmas/Easter/St Patrick’s Day and for Back to Our Past in Dublin in October.
Ethnicity estimates are provided by all companies. This consists of a break down of your overall geographical ancestry. These results change over time as they get more people in their database and revise markers. These results should be considered just a bit of fun and shouldn’t be taken too seriously. They also will vary from company to company depending on who is in their database. At the moment, they are only considered accurate on a continental level, but this is also expected to improve over time.
Adoptees have started to make significant breakthroughs in finding birth parents through the use of DNA testing. If you can make contact with someone who is a close match, i.e. second cousin or closer, then they may well be able to help solve your mystery.
All takers of DNA tests should be prepared for the possibility that these results may reveal unexpected things including but not limited to an extra sibling or discovering your father is not your biological father. This story in the Washington post appeared last month. It's a long but very interesting read on a DNA discovery.
Gedmatch is a free third party website which allows you to upload results from all major commercial sites. Follow instructions on their site to upload results. For $10, they have extra tools and applications which include Lazarus kits – if you have a child and sibling of a deceased person, you can extrapolate their DNA, it works best if you have tested the child’s other parent too.
Back to Our Past runs a lecture strand on DNA testing each year called Genetic Genealogy Ireland and a lot of the previous years' lectures are on YouTube. I recommend watching some of their stuff if you are a total newbie.
This article published August 2017 is a very good detailed breakdown of the current tests, albeit a little US centric.