Previously, when the number of online users was limited, the search engine result pages were very basic and limited to just the search results.
  However, as the number of online users increasing drastically, the content of SERP is also changing drastically.
  Now we get lot of advertisements related to what we are searching.
It has become new tools for advertisements.
All ranges of advertisements are displayed, right from the services to products.
  Order of search engine result page is based on content found of search item and most viewed related website.
This has made every one life easy.
    The contents of an SERP include the search results pertaining to the search made.
  It also includes other suggestions from the search engines, even these are generated relating to the keywords pertaining to the search and the rest are the advertisements usually from the sponsors, even these will be related to the search.
Sometimes, an SERP may also contain suggestion from the search engine to find out if you mean something like this.
  For example, if you type in a wrong word or a spelling mistake, Google asks 'did you mean so and so'.
   The contents of an SERP will not remain the same for long.
  They are changing as per the requirements of the users.
  For example, now most of the search engines are also providing directions to the point, if the user is looking for a place.
Accordingly, as people are getting more adapted to the technology, the search engines are trying to make things easy for the users by including more useful things are being added to the content of an SERP.