There is a plinth at each corner of Trafalgar Square. The two plinths on the south side carry sculptures of Henry Havelock and Charles James Napier. The northeastern plinth, carries an equestrian statue of George IV.
The fourth plinth on the northwest corner was intended to hold an equestrian statue of William IV, but remained empty for lack of funds. In 1998, the Royal Society of Arts came up with the “Fourth Plinth Project”, designed to bring a variety of contemporary works to Trafalgar Square. Although this initial project was felt to be successful, the plinth remained empty for some time afterwards. Eventually, however, the new Greater London Authority assumed responsibility for the Fourth Plinth and started its own series of changing exhibitions.
The current work is Hahn/Cock (2013) by German sculptor Katharina Fritsch.
Here is a short video featuring the Mayor of London unveiling the blue cockerel and some initial reactions to it.