How does Google use HTML tags to enhance the search engine?

Question

I know that Google’s search algorithm is mainly based on pagerank. However, it also does analysis and uses the structure of the document H1, H2,

Answer 1

I would also suggest looking at Microformats and RDF's. Both are used to enhance searching. These are mostly search engine agnostic, but there are some specific things as well. For google specific guidelines for HTML content read this link.