Chimerism results when two or more eggs become fertilized and then fuse together. A human chimera has at least two types of DNA in their body; this person is essentially their own twin (triplet siblings, etc). However, chimerism is much more prevalent in animals than humans.
The term 'chimera' is also used with regard to recombinant DNA. In that sense, a chimeric DNA molecule results from enzymatic joining of DNA from one species (e.g. human) to that of another species (e.g. a cloning vector derived from a bacterial plasmid).