From ISOGG Wiki
The so-called CODIS markers are a collection of thirteen autosomal STR markers. The "CODIS" name refers to the Combined DNA Index System, which is a database in the United States consisting of profiles derived from the so-called CODIS markers.
Because of their autosomal nature, CODIS markers can be used to assess whether two people are likely to be very closely related to each other. In particular, of the 26 alleles that comprise a person's CODIS markers, 13 would have been inherited from each parent. Two siblings will, on average, share half of their 26 CODIS alleles in common with each other.
CODIS DNA tools can be used to take a person's CODIS markers and determine some sense of the person's ethnic background. To do this they generally perform a comparison with CODIS markers collected from various sample populations.