Recently I convinced several cousins to test their DNA at 23andme since the price is now only $99 – 23andme is pushing to get one million subscribers. The idea is that by having a large enough database with subscribers that answer their health and trait surveys, correlations can be found with the genes responsible. 23andme has already contributed greatly to the current knowledge of DNA using this technique. So I feel particularly good about being a part of that. Click here for the list of correlations that they have so far.
What they do is not a complete genome sequencing, just the markers that are most likely to be different from one person to the next. Remember we share about 98.5% of our genome with chimpanzees and 99.9% with other humans. These tests use a microchip array that actually tests about .o2% of your genome.
Family Tree DNA has an excellent FAQ that explains the details of this type of testing. They use the same Illumina chip as 23andme so much of this is valid for that test also even though 23andme tests some extra variants.
23andme also has this page on DNA basics:
But I send all my cousins and family to my own 23andme tutorial posted here on this site.