Latent Semantic Analysis (LSA) is based on the well known Singular Value Decomposition Theorem from Matrix Algebra but applied to text. That is why some of the semantic analysis that is done at the page content level it may also be done on the linkage data.
LSA represents the meaning of words as a vector, thus calculating word similarity.
SEOs have a similar goal. They try to discover which words and phrases are most semantically linked together for a given phrase, so when Search Engines crawl the web, they find that links to particular pages and content within them is semantically related to other information that is currently in their database.
for more infor got there:
http://www.seobook.com/archives/000657.shtml