Proposition 8, better known as Proposition H8te, was a ballot proposition in California. There were numerous demonstrations and counter demonstrations. There was even violence. The voters ruled that marriage should only be between a man and a woman. The American courts decided the legality of all this and in fact, it was declared unconstitutional by Chief Judge Vaughn Walker in his ruling of Perry v. Schwarzenegger,  the Supreme Court later upheld this and Gay marriage was restored in California. 
This proposition isn't all that important, both gays and straights will have their fun either way, it's no big deal. But people focused on this unimportant proposition about 10 times all others combined. The high speed train and redistricting props (for example) were far more important.
The proposition's ad campaign was a waste of cash and voters ended up deluded into thinking "no" would do something. But NO means nothing happens.
America and Britain compared
In Britain for years gay people could legally register same sex partnerships. The Gay community and the straight community frequently called this Gay marriage. Gay people in same sex partnerships had similar rights and responsibilities to men and women in heterosexual marriages. Just the law called it something different. The issue was far less controversial than it is in America.
In America the issue is covered in terms of the Politics of whether gays should or should not be allowed to marry. In Britain the coverage mostly focuses on the bling factor and other gossip pertaining to celebrity gay weddings such as that of Elton John.
In America there is a legal difference between the two terms
What non-U.S. citizens might not understand is that in the United States, "separate but equal" categories under the law are illegal as decided in the Supreme Court case of Brown v. Board of Education (1954) -- specifically the court ruled "separate but equal is inherently unequal". They also might not understand that in the U.S., state and/or local laws that apply to marriage can exclude civil unions on the naming technicality, and that "civil unions" are not recognized by the federal government whereas "marriages" are. The reason this is such a contentious issue over "just a name" is because of the unique relationship between state and federal law in the United States.
To further illustrate the differences between a "civil union/domestic partnership" and a "marriage" and to understand why it is so important to understand the distinction, please see the following videos.
These latter two deal with the more specific legal issues that surround marriage in the United States.
- "Prop 8 - The Musical" starring Jack Black, John C. ReillyThis video is really Cool.