There are several guard units, and the Beefeaters are one of them. Here's the scoop, and the list of guards from Buckingham Palace's website at http://www.royal.gov/uk
"CHANGING THE GUARD
The regular hand-over of guards in the forecourt of Buckingham Palace draws thousands of visitors to watch the soldiers and listen to the music.
Known as Changing the Guard or Guard Mounting, the process involves a new guard exchanging duty with the old guard.
The soldiers are drawn from one of the five regiments of Foot Guards in the British Army: the Scots Guards, the Irish Guards, the Welsh Guards, the Grenadier Guards and the Coldstream Guards.
The handover is accompanied by a Guards band. The music played ranges from traditional military marches to songs from the shows and even familiar pop songs.
When The Queen is in residence, there are four sentries at the the front of the building. When she is away there are two.
The Queen's Guard usually consists of Foot Guards in their full-dress uniform of red tunics and bearskins. If they have operational commitments, other infantry units take part instead.
Units from Commonwealth realms occasionally take turn in Guard Mounting. In May 1998, Canadian soldiers from Princess Patricia's Canadian Light Infantry mounted guard at Buckingham Palace for the first time since the Coronation in 1953.
Household Troops have guarded the Sovereign and the Royal Palaces since 1660. Until 1689, the Sovereign lived mainly at the Palace of Whitehall and was guarded there by Household Cavalry.
In 1689, the court moved to St James's Palace, which was guarded by the Foot Guards. When Queen Victoria moved into Buckingham Palace in 1837, the Queen's Guard remained at St James's Palace, with a detachment guarding Buckingham Palace, as it still does today."
There is no Guard Mounting in very wet weather.
Buckingham Palace
October - even days
November - odd days
December - odd days
Windsor Castle
Changing the Guard takes place at 11.00am.
October - odd days
November - even days
December - even days
Horse Guards Arch
Daily at 11am.
Tower of London
Daily at 11.30am.
At Buckingham Palace, Guard Mounting takes place at 11.30 am. It is held daily from May to July, and on alternate dates throughout the rest of the year.
Buckingham Palace is not the only place to see Guard Mounting.
At Windsor Castle, the ceremony takes place at 11.00 am. For most of the year Guard Mounting takes place on alternate dates, but it is held daily (except Sundays) from April to July.
At Horse Guards Arch, Changing of the Guard takes place daily at 11.00 am (10.00 am on Sundays) and lasts about half an hour; it is normally held on Horse Guards Parade by the arch of Horse Guards Building.