What is the legal difference between a civil union between homosexuals
versus marriage of homosexuals.  Why would homosexuals care about
marriage versus civil union.

The attached article should give you some of the information you are seeking:
The Difference Between Marriage and Civil Unions
 From Kathy Belge,
http://lesbianlife.about.com/cs/wedding/a/unionvmarriage.htm

Here is a chart that breaks down analysis:
http://www.glad.org/rights/Marriage_v_CU_chart.pdf

If you do a search for "civil union vs. marriage" you'll see these and
other resources.