Hello Mr Scot - Can you just briefly explain to me what the key jobs are for a GM? There are lots of different roles within an organisation and i'm not entirely sure who does what.
I'll give you what I know.
The GM's primary job is personnel evaluation, deciding who's worth drafting, who's worth signing in free agency, which players currently on the team deserve contract extensions (for how long and how much)...essentially acquiring the "groceries" as Bill Parcells put it that the head coach uses to set his roster.
Most GMs these days oversee the cap as well, though often it's with the assistance of a cap manager (someone with strong accounting skills). It's tough to find one guy who's great at both scouting and accounting, so generally the GM sits down with the cap guy to work things out. The GM might say what he thinks a certain player is worth, to which the cap guy replies with what the team can afford.
Although the general structure might vary somewhat from team to team, in most cases the GM's desk is where the proverbial buck stops. He oversees everything related to the football side of things, including stuff like the hiring and firing of front office people (and sometimes coaches) and oversight of the scouting departments, the ones responsible for gathering the information that the GM uses to make his player decisions.
So yeah, overall it's a really big (and extremely difficult) job.
Hope that helps.