This looks like a very straight forward question, but the answer is very complex. I would suggest three or four books to give an idea of the complexity of the topic. These are (all on Amazon, UK for ease of access) http://www.amazon.co.uk/Soldiers-French-Revolution-Bicentennial-Reflections/dp/0822309351/ref=sr_1_5?ie=UTF8&qid=1428588055&sr=8-5&keywords=forrest+revolution http://www.amazon.co.uk/Conscripts-Deserters-French-Society-Revolution/dp/0195059379/ref=sr_1_4?ie=UTF8&qid=1428588055&sr=8-4&keywords=forrest+revolution http://www.amazon.co.uk/Response-Royal-Army-French-Revolution/dp/0198225342/ref=sr_1_1?ie=UTF8&qid=1428588229&sr=8-1&keywords=line+army+french+revolution There is another which I can't remember at the moment; there's a copy in my office at work, where I won't be until Monday. The other ranks of the infantry tended to be made up of the desperate urban poor, the other ranks of the cavalry, the rural poor, and the artillery from both. All were long term enlistees who had joined through desperation, poverty, or disinclination to normal life. Their pay was poor as was their conditions. Occasionally the Militia could be draughted into the line army, often causing riots. The officers were a mixture. By a recent law, St.Germain's I think, officers had to be noble, but many, in the infantry at least, weren't. The higher nobility gravitated towards the cavalry, the hobereaus (poor rural nobility) tended towards the infantry, where expenses were less. The artillery officers were sons of ennobled merchants and lawyers and the lesser nobility. They had to have a higher level of education for their craft, and had usually attended one of the military colleges, like Brienne. There were officers in the infantry, sometimes described as "routiers" who had been promoted from the ranks. These were always of low rank, rarely rising to captain, and almost never higher. They tended to stay with their regiment when the more wealthy officers went on frequent leaves, especially if the regiment was stationed far from Paris! There were also several "foreign" regiments, mostly German and some Swiss, who were mercenaries, and who remained absolutely loyal to the Bourbons. The army fought for the King (I'm disregarding the Garde and the Maison du Roi for now) before and during the Revolution, and remained loyal to the Constitution, and the Constitutional monarchy after its establishment in 1792. With the Revolution many of the noble officers emigrated; the new government had abolished the requirement for nobility, which allowed many of the more able NCOs to gain commissions, people like Ney and Murat, for example.

Which one?

As Second_Mate points out, these are not easy questions, and he's given you some good readings. Forrest really is the most prominent anglophone scholar on the revolutionary armies, but there are also others, including Germani and Blanning, etc.

A few key points, nonetheless. I think the other suggestions here are wise to describe what they have, but it should be pointed out that there were many different stages of military composition throughout the Revolution as policies and indeed political power itself shifted. Even at the Revolution's beginnings, there were a myriad of varied armed forces: there was the army, of course, but also the Gardes françaises and Gardes suisses, the National Guard (established just days before the Bastille by Lafayette), and the milice bourgeoise, for Paris alone. When troops fired upon petitioning republicans in July 1791 in what became the Champs de Mars Massacre, it was Lafayette's National Guard and not the army itself that was responsible. Which is important, too -- the resultant reputational decay of Lafayette was not imposed elsewhere, instead fixing antagonism upon him and the National Guardsmen too; see, e.g., Andress (2000) on this.

And of course, as others have pointed out, the role of the levée en masse can't be denied. Once introduced, it radically changed the composition and, with it, entailed various disciplinary problems; and nor can Parisian politics be detached from the realities of military life in this regard. Hierarchy, for instance, flew in the face of the Revolution's increasing calls for equality. As early as 1791, debates were being had over such structures in the National Guard as opposed to the army, whether to impose different standards on either (as they mostly eventually did), how insubordination might be avoided while fostering 'revolutionary' values in the army ranks, etc. In terms of military fronts, finally, these were numerous and again it depends on the chronology. There are plenty of maps online that helpfully if sometimes crudely depict the various fronts at play throughout the Revolutionary Wars, e.g. https://herodotethourioi.files.wordpress.com/2014/02/france__1793_carte_simplifiee_10_annexions.jpg.