how does postgres handle full text search code example

Example: postgresql full text search

Full Text Searching (or just text search) provides the capability to 
identify natural-language documents that satisfy a query, and optionally
to sort them by relevance to the query. The most common type of search is
to find all documents containing given query terms and return them in
order of their similarity to the query. Notions of query and similarity 
are very flexible and depend on the specific application. The simplest
search considers query as a set of words and similarity as the frequency
of query words in the document.