Short answers, pulled from the story.
The Old Guard was a formation of veteran troops within the French Army's Imperial Guard, considered the most prestigious formation in Napoleon's Grande Armée. French soldiers called them "the Immortals," and Napoleon called them "my children."
Recruits had to be under 35 years old, have at least 10 years of service and at least three campaigns, and have faced enemy fire at the front. Height requirements were 5 feet 8 inches for Chasseurs and 5 feet 10 inches for Grenadiers, waived for anyone awarded the Legion of Honor.
The Old Guard Grenadiers were nicknamed les Grognards, the Grumblers, because they were free to openly complain about the petty troubles of military life. Captain Jean-Roch Coignet traced the name to hardships suffered during the War of the Fourth Coalition, though some say Napoleon himself coined it.
The Old Guard had four infantry regiments, a 1st and 2nd each of grenadiers and chasseurs, and four cavalry regiments: the Grenadiers à Cheval, Chasseurs à Cheval, Dragons de l'Impératrice, and the 1st Polish Lancers. The Mamelukes of the Imperial Guard squadron and the Gendarmes d'élite were also counted as Old Guard cavalry.
At Waterloo in June 1815, the 2e Régiment de Grenadiers-à-Pied was pivotal in defending the village of Plancenoit against the Prussians. The 1er Régiment served as a rear guard after the Middle Guard's attack on the Anglo-allied center failed, and the Old Guard cavalry's midday charges failed and were unavailable at the battle's decisive moments.
The Sixth Coalition disbanded the Old Guard in 1814 along with the rest of the Imperial Guard, but it was reformed after Napoleon's 1815 return from exile. Louis XVIII ordered the Imperial Guard abolished in August 1815, and by December of that year every Old Guard regiment had been disbanded.