Autosomal DNA or atDNA pertains to the genes or genetic markers in any chromosome other than the sex chromosomes (XX or XY). In humans, autosomes are the set of chromosomes labeled 1 to 22. Chromosome 23 is the sex chromosome. See the chart below, Chromosomes in Human DNA. Also here is a link to a video about about Autosomal DNA at Sorenson Molecular Genealogy Foundation and here is a link to a discussion by Dr. Erin Cline Davis of 23andMe.
Since autosomal DNA or the atDNA is made up of many random combinations of genetic blocks of information, its uses in genealogy have been limited thus far.
Because all atDNA does not combine in a definitive pattern, some atDNA is inherited in a larger quantity than other atDNA. For example, a son may inherit more atDNA from his mother than from his father.
As it was stated above, the "recombination" of atDNA is random. Scientists still do not completely understand the random "recombination," but have been developing theories on it.
To illustrate this random "recombination" of atDNA, see the chart with colored graphics below called, Autosomal DNA Random Recombination.
In the chart, color is used to show the atDNA of two children and their direct ancestors, mother, father, grandparents, etc. The oldest generation is shown with a solid color, that is randomly "recombined" in the child of the next generation and so forth until the parents of the two children are shown at the bottom. The two children, a brother and sister, contain all the randomly "recombined" atDNA of all their ancestors.