Robots - When a spider from a search engine reaches your site, you can actually instruct the spider to index content in different ways by using the Robots drop down menu in Joomla! 2.5.
Index, Follow = the spider will index the entire site - the front page and every other page on the site.
No Index, Follow = the spider will not index the front page, but will index every other page on the site.
Index, No Follow = the spider will index the front page, but will not index any other page on the site.
No Index, No Follow = the spider will not index the front page and will not index any page on the site.
Please note that just because you tell a search engine to index some content and not other content, it may not pay attention to you. Trusted search engines will understand this code and will follow directions, but other search engines may not be able to understand the code or simply may not care.