A search based on frequencies of the search terms in the documents being searched. Weighted search is often used by search engines. It produces a numerical score for each possible document. A document's score depends on the frequency of each search term in that document compared with the overall frequency of that term in the entire corpus of documents. A common approach is called tf.idf which stands for term frequency * inverse document frequency. Term frequency means "the more often a term occurs in a document, the more important it is in describing that document." http://ciir.cs.umass.edu/cmpsci646/ir4/tsld034.htm Inverse document frequency means the more documents a term appears in, the less important the term is.
For a detailed technical discussion see Chapter 5, "Search Strategies", in the reference below.