Trooping the Colour is a ceremony performed by regiments of the Commonwealth and the British Army. It has been a tradition of British infantry regiments since the 17th century, although the roots go back much earlier. On battlefields, a regiment’s colours, or flags, were used as rallying points. Consequently, regiments would have their ensigns slowly march with their colours between the soldiers’ ranks to enable soldiers to recognise their regiments’ colours.

Since 1748 Trooping the Colour has also marked the official birthday of the British Sovereign.[1] It is held in London annually on the second Saturday in June[2] on Horse Guards Parade by St James’s Park, and coincides with the publication of the Birthday Honours List. Among the audience are the Royal Family, invited guests, ticketholders, and the general public. The colourful ceremony, also known as “The Queen’s Birthday Parade,” is broadcast live by the BBC.

from http://en.wikipedia.org/wiki/Trooping_the_colour

Please find the audio slide show for 2010 event God Save the Queen.