A Barr body is a condensed, inactive X chromosome found in the somatic (non-gamete) cells of normal females and genetically abnormal males who have more than one X chromosome. Barr bodies are visible when the nucleus of the cell is not dividing.

None of the genes in a Barr body are used, since only one X chromosome can function within a cell. In every cell with more than one X chromosome, one is randomly turned off to create a Barr body. Because of this phenomenon, identical twin girls display more phenotypic differences than identical twin boys.

From the science dictionary at http://biotech.icmb.utexas.edu/