The Poliziottesco was a sub-genre of film that became popular in Italy during the 1970's. They were crime/action films produced mainly in Italy and were heavily influenced by American crime films of the time, such as The Godfather, Dirty Harry, The French Connection, and so on.
The best source for learning more about this sub-genre is the fantastic documentary, Eurocrime! The Italian Cop and Gangster Films That Ruled the '70s, directed by Mike Malloy.
The ten films listed below are, in my humble opinion, the best films to start with if you are just getting into the genre.