I'm a 6ft, 165lbs guy and have been trying to gain muscle mass for a while but just haven't had much luck...
I go to school and work so I can only really fit in 2 hour long weightlifting sessions/week (I know a lot of you guys would say that isn't nearly enough).
As far as diet... protein supplements are way to expensive for me, so I have been trying to snack more thoughout the day (usually with decently healthy foods) and drink a lot of chocolate milk.
The most I've gained is 5 lbs and that is even hard for me to keep on...
So, I've been thinking aboutjust focusing on toning up the muscles I already have instead of trying to bulk up. How would a toning up routine (diet and exercise) differ from a bulking up routine?

I don't follow what you mean by toning opposed to bulking. Lose fat vs. building muscle? Maintain muscle you already have? I suppose higher rep, lower weight lifting will help to maintain muscle tone and as an aerobic workout burn some fat also.

Just fyi, if you meant that someone had told you that you have to lift in 2+hour sessions at a time, that's horribly excessive. 2 hours a week is less than I would recommend, but certainly do-able. You can get a sufficient workout in around 30 minutes if you lift smart.

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Keep your body lean, your blood clean and your mind sharp. -Rollins

Ok.
I intially wanted to gain muscle mass ( and still do ) and I know it's doable for anyone, but I'm such a hard gainer that I just don't see me gaining that much mass, which is a bummer.
By toning I mean maybe putting on a little muscle mass, but making that muscle more defined, more visible - I don't know if I'm explaining it right. I definitely don't want to lose weight, but could I maintain my current weight and just make my muscles more defined?
I meant many people would say I wasn't working out enough days/week. One hour per session is ok, but two days a week probably isn't the best routine.

Cardio coupled with weights. A proper diet to lose the layer of bodyfat (I don't mean "diet" I mean eating right).

Weight routine should be a mixture of high rep/lower weight and high weight/low reps. A pyramid might be ideal for you. Consistency is key.

Hopefully, Canadian Convict will respond to this post...he knows what to do for this and has a good routine. You could probably search the board for a post he made about his routine. It is a good one.

What exactly is a pyramid?