GEDmatch is a free website.
(there are few more tools available for a subscription option, but not neccessary)
The goal of the website is to be able to help identify your DNA matches and which segment(s) of the Chromosome(s) you match.
From there you can identify others sharing the same segment(s), and hopefully from there you can identify a shared ancestor, and from there attribute that segment of DNA to that ancestor, which then helps you work out how your other matches match.
All a bit complicated?
Ancestry shows you only your matches, it does give you an idea of how much DNA you share, according to their algorithm, and a suggestion of how close that relationship is.
For example
GEDmatch will pin down this information more precisely allowing you to find your cousins easier.
For example, here is the same match after running a "One-to-One compare"
It tells me which Chromosome and where, how long and how many SNPs match, and gives an estimate of how many generations back to the MRCA - "most recent common ancestor".
From here I can run a "People who match 1 or both of two kits" and this will show me if anyone matches the same segment, in the example below I have now found 4 others who appear to share the same DNA segment.
Now, to be sure that you all share the same ancestor you will need to run the "One-One compare" for each match on the list to each other match. Some may not actually match the others - this is because for each SNP we have 1 part paternal and one maternal, so the matches that don't match each other will be from your other side.
Once you have found a group of 3 or more (distant from each other ie not including close family members) that do match, then this is called a Triangulation Group. From here it comes back to the paper trail to verify the actual connection. So you will need to have a properly proven tree back as many generations as possible.
Sound good?
So, how do you get onto GEDmatch...
Firstly login to your Ancestry account and go to your DNA Results Summary.
On the right hand side of this page is a grey box with cog icon marked Settings.
Click this and then you can click on Download Raw DNA data
You will be sent an email with a link, save the file somewhere you can find it.
Now, go to www.GEDmatch.com and register for an account
Now you will have a start page with the following
Click on the AncestryDNA.com option, and upload your file.
You will be given a kit number, take note of this.
It will take some time for your matches to load (4hours-2days), but if you know any other person's kit number you will be able to run a one-to-one compare whilst you wait.
Also a good idea is to upload your gedcom file, and read the help files.
Once your file is "tokenised" you can run the One-to-many matches
You will see something like this
Your matches are ordered from best to worst
By clicking on the "A" button this will take you to a one-to-one compare and you can go from there - as above.
As a guide you need to be looking first at those who share more than 10cM, and more than 1200 SNPs.
Matches can be false as DNA is totally random, so the higher the cM and the SNPs the more likely it is to be a real match. Anything higher than 15cM is pretty definite, or 1800 SNPs for a 10cM match.
For a 7cM match about 1500 SNPs is around 50% likely of being a true match.
Good luck!
Here are some links that can guide you further
http://smithplanet.com/stuff/gedmatch.htm
http://www.genie1.com.au/blog/78-tips-for-using-gedmatch
