What is Viewpoint Discrimination?

How is viewpoint discrimination different from content discrimination?

Is the subject or topic the speaker wishes to address permitted? If so, is the speaker being excluded from the forum only because of the viewpoint or ideological perspective from which he or she wishes to address the subject?

Here is a great quote from Prof. Michael McConnell on viewpoint discrimination:

"In my opinion, whether a restriction is viewpoint discriminatory [depends upon the answer to the following] realistic question: Are there any identifiable ideological groups of thought that are put at a disadvantage relative to their competitors? When religious speakers are excluded (even if 'religious' includes atheists), their perspective is put at a disadvantage vis-a-vis dozens of other competing worldviews. Thus, a person who thinks the welfare reform bill is bad because it is contrary to Marxist theory can get government money and proclaim his views, but the person who thinks it is bad because it is contrary to Christ's admonition to feed the poor cannot."