Held daily at Buckingham Palace, the Changing of the Guard is a popular attraction for tourists visiting London. The ceremony allows the old guard to leave their posts and installs the new guard in place. Each guard is a soldier in the Household Division, the part of the miliary charged with guarding and serving the Sovereign. Guarding the palace has been one of the Household Division's roles since the late fifteenth century, when Henry VII began the practice. In addition to lining the front of Buckingham Palace, tourists often view the parade from the Wellington Barracks and St. James Palace; climbing on the Victoria Memorial, however, is forbidden. Other popular ceremonies involving British royalty include Trooping the Colour for the Queen's birthday, and the Ceremony of the Keys held nightly at the Tower of London. A guidebook is available for those interested in these ceremonies, and the Guards Museum and Household Cavalry Museum are usually open to the public.

Another Changing of the Guard popular with tourists is that at the Tomb of the Unknown Soldier at Arlington National Cemetery. From the beginning of October through the end of March, an elaborate ceremony is conducted every hour on the hour; from the beginning of April through the end of September the guard changes every thirty minutes and the cemetery closes two hours later. Each changing begins with the relief commander appearing on the plaza, and then the sentinel leaves the Tomb Guard Quarters and unlocks the bolt of his or her M-14 rifle to signal the relief commander the ceremony is ready to begin. The commander walks out to the tomb and salutes, then faces the spectators and asks them to remain standing and silent throughout the proceeding. He or she then conducts a white glove inspection of the sentinel's rifle and the pair walk to meet the retiring sentinel at the center of the path in front of the tomb. All three salute the Unknowns, and the relief commander instructs the departing sentinel to "Pass on your orders." The sentinel gives the command "Post and orders, remain as directed" and the incoming sentinel responds with "Orders acknowledged" before stepping into position. The Tomb Guard marches 21 steps down the black mat behind the tomb, turns to face east for 21 seconds, turns to face north for 21 seconds, and then takes 21 steps down the mat to repeat the process. After the turn, the sentinel executes a shoulder arms movement, signifying that they stand between the tomb and any potential threat. The 21 steps and 21-second pauses are representative of the twenty-one gun salute, the highest military honor that can be bestowed.

Several other similar ceremonies take place around the world, including at Warsaw's Tomb of the Unknown Soldier in Warsaw and at the Martyr's Shrine in Taiwan, and on Ottawa's Parliament Hill.

Sources
http://www.changing-guard.com/
http://london.travelape.com/attractions/changing-of-the-guard/
http://www.arlingtoncemetery.org/ceremonies/sentinelsotu.html