They rely instead on key words. Search engines can only find words that are actually entered in your query. This often results in hits that are completely irrelevant to your query. In order to search effectively, you need to understand how to refine your search by combining search terms. The basic Boolean operators AND, OR and NOT are used to combine terms in most search engines.
Many search engines default to AND if a string of words is typed in.
OR retrieves documents containing any of the terms you enter, but not necessarily all. It is useful for searching synonymous terms. Germany OR Deutschland will find documents using either word. NEAR retrieves documents with the search terms appearing within a specified number of words of each other. Cancer NEAR horoscope would find information about the astrological sign Cancer, but likely not information about the disease.