During earlier times, all websites used static HTML pages and so the first search engines were oriented towards static web pages. As the web technology developed several new methods to generate websites and dynamically generated web pages came into being. Dynamic web pages differ from static pages in that there is no physical equivalent to the page you may be viewing on the server.
Dynamic URLs are generally used if the content of a site is stored in a database and pulled for display on pages on demand.
But it is said that URLs have a distinct disadvantage in that different URLs can carry the same content.
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A simple example would be a 1000000 page site with each page generated with the format below
http://fishbase.org/Country/CountrySpeciesSummary.cfm?c_code=356&id=24516
http://fishbase.org/Country/CountrySpeciesSummary.cfm?c_code=356&id=24517
http://fishbase.org/Country/CountrySpeciesSummary.cfm?c_code=356&id=24518
http://fishbase.org/Country/CountrySpeciesSummary.cfm?c_code=356&id=24519
http://fishbase.org/Country/CountrySpeciesSummary.cfm?c_code=356&id=24520
http://fishbase.org/Country/CountrySpeciesSummary.cfm?c_code=356&id=24521……
All the above mentioned six URLs end up with six different landing pages. But when the search engine tends to remove the information subsequent to the first offending character, the question mark (?), ultimately all the six pages become identical :
http://fishbase.org/Country/CountrySpeciesSummary.cfm
http://fishbase.org/Country/CountrySpeciesSummary.cfm
http://fishbase.org/Country/CountrySpeciesSummary.cfm
http://fishbase.org/Country/CountrySpeciesSummary.cfm
http://fishbase.org/Country/CountrySpeciesSummary.cfm
http://fishbase.org/Country/CountrySpeciesSummary.cfm
At this instant, your unique pages have become identical, and consequently the URL’s are duplicated which won’t be indexed by the search engines.