I heard a short show on this on NPR this morning. It's fascinating.
Basically, it seems, two separate fertilized human embryos which “should” have become fraternal twins, instead fuse together and form one child. This child receives BOTH sets of genes from the two different embryos. Until DNA testing was somewhat common, this went completely undetected. True human Chimeras can pass on either set of genes to their children, while they only have one set specifically in their blood (so paternal blood tests might have false negatives). Or they could leave traces of one set of DNA at a crime scene and have the other set show up in testing.
That page has a link to a recent article in Nature mag, too.