Military history is definitely not my thing!
Working through different combat logs and accounts from the 1840s and while I know the difference between a matelot (sailor) from a grenadier, tirailleur, or voltigeur; I am not sure exactly what differentiates that last group? In this case all four are serving together.
Obviously the difference is important enough for the sources to usually be clear about what type of soldier/unit to which they refer (though not all the time).
Was the training different? Command structure? Responsibilities? Armaments?
Thanks in advance for any and all input.
*edited to add voltigeur to the list; forgot it the first time!
Partially it's historical tradition--if a soldier is assigned to a unit with "grenadier" in the name, he's called a Grenadier. Similar to how modern tank units are still officially called "cavalry," despite no longer riding into battle on horseback.
Initially, Grenadiers carried grenades--hence the name. By the 1840s, though, designated grenade-throwers had been phased out. Militaries are anything if not big fans of tradition, so units that had originally been grenade-throwers retained the name. Often, Grenadier units--even late in their history--selected or recruited the biggest, toughest guys they could get a hold of. By 1840, the "grenadier" epithet was frequently used for elite line infantry units.
Tirailleurs and Voltigeurs were both light-infantry skirmishers. Skirmishers were trained to range out ahead of the main body of the army to harass, scout, and disrupt the enemy--and prevent the enemy from doing the same in return. While the heavy line infantry were armed with muskets, light infantry skirmishers were most frequently armed with rifles or rifled carbines.
The Voltigeurs were so named due to their original intended mode of deployment--to "vault" onto cavalry horses behind the rider, and be carried forward by their cavalry comrades. This didn't work out so well, so they reformatted into skirmishers like the Tirailleur. Being tradition-bound, they kept the name. They were a peculiarity of the Napoleonic area, and by 1840, the Voltigeurs had long since been disbanded.
To get at part of your unasked question about deployment, yes, grenadiers, line infantry, and skirmishers would all be serving together, but in separate companies. Generally, an army would be made up of divisions, which were made of brigades, which consisted of regiments or battalions (depending on who's army we're talking about). Battalions would be made up of companies. In an infantry battalion, most of these companies would be line infantry, at least one would be a skirmisher company, and one or two could be grenadier companies (the elites of the battalion).
Grenadiers were typically give more ceremonial roles and were considered crack troops.
They wore distinctive and often elaborate uniforms, and in some armies were picked from the infantry due to their tall/intimidating stature/
I don't really know anything specifically about French Grenadiers though.