Semantic search attempts to retrieve information based on the searcher intent and the contextual meaning of terms entered by the searcher. This is ideally suited to searches where the user is trying to gather or research general information on a topic. In this instance, the user is trying to locate a number of documents which together will provide the necessary information.

Rather than using ranking algorithms such as Google's PageRank to predict relevancy, Semantic Search uses semantics, or the science of meaning in language, to produce highly relevant search results. In most cases, the goal is to deliver the information queried by a user rather than have a user sort through a list of loosely related keyword results.

http://en.wikipedia.org/wiki/Semantic_search