I was reading John Keegan’s Face of Battle and was a bit confused by the references to the different types of cavalry. He referrs to heavy cavalry, light cavalry, the “union brigade” of British cavalry, dragoons, and cuiraissers.
Can any Doper sort out these terms for me, and their significance for the role the different units played in combat.
(And, did the Union Brigade sing “Solidarity Forever” before setting off on a charge?) 
It generally boiled down to: duties, armament, tactics, and size of rider. Smaller men on smaller, faster horses served as scouts, skirmishers and messengers. Larger men on larger horses served as shock troops.
What Silenus said about light and heavy cavalry. Dragoons were originally mounted infantry but by the early 19th Century had become cavalry of all tasks and were divided into heavy and light dragoon regiments. The equipment and arms of heavy and light Cavalry were slightly different but they all relied on a sword as their principal weapon. At Waterloo British Heavy Cavalry did not have breastplates but they did wear a metal helmet with a quasi-classic crest.
The Union Brigade was so called because it was made up of heavy dragoon regiments which were nominally Irish, Scottish and English, the three countries that made up the United Kingdom of Britan and Ireland by virtue of the Act of Union. The regiments were the First Dragoons called the Royal Dragoon Regiment (English), the Second Dragoons called The Royal North British Regiment and the Grey Scots (Scottish), and the Sixth Dragoons, called the Inniskilling Dragoons (Irish).
The other British heavy cavalry brigade at Waterloo was the Household Brigade composed of the First and Second Life Guards, the Royal Horse Guards (The Blues), and the Dragoon Guards. These are the outfits that provide the ceremonial mounted guards at Buckingham Palace, White Hall and Horse Guards’ Parade.
Interestingly enough the units that made up the Union Brigade at Waterloo were in the Heavy Cavalry Brigade at Balaclava in the Crimea. When on October 25, 1854, the Heavy Brigade attacked to repel a Russian attempt to cut the British army off from its supply base the attack had to be delayed while the Greys and the Inniskillings arranged themselves as they had been at Waterloo some forty years before. Note that this was NOT the more famous and much less successful Half a League, Half a League, Half a League Onward action of later that same day. The charge by the Heavy Brigade followed the “Thin Red Line” thing and proceeded the Charge of the Light Brigade.