Conscription aka forced service wasn't a Napoleonic invention. Massed conscription however, meaning that basically every able-bodied male citizens could and would be called up, was a whole new thing.
It was quite an interesting system though, that Napoleon used. You'd get a number, say, for example, 53, and they'd designate the 'call-up' for your specific year at something like 40, meaning that all the people who drew a number 40 or lower were immediately called up. If, in a later stage, more men were needed, they'd simply raise the number and all those under the new number were also called up.
You could switch numbers - many impoverished people who would draw a high number would trade it for a low one in exchange for a sum of money. And these were very serious amounts of money, which would often enable the soldier to start a farm or a business when he came back. If you had brothers already serving in the army, you would get basically get a 'get-out-for-free'-card, meaning you could never be called up. These 'cards' could also be traded, and the prices paid for those were extremely high - because it meant you had 100% certainty you would never have to serve.
There were some other reasons that would exclude you from service, like being the sole child of your parents, to make sure the conscription didn't have to much of an impact on society.