It's more than that.
Volunteers: are people that want to help out at con, and before con, but not necessarily make the "leap" to staff. They come to the con for events first, and helping out second. They are often the people that are bored during "lull" times, and want to help out. Volunteers are generally 15+.
Staff: are the people that want to help the con go places. They come to the con to help first, and go to events second. People have this idea that staff work all the time and don't get time to enjoy themselves. That's partially true. Staff work a lot. But part of that is because there aren't enough staff. If we could all convince more friends to be staff or volunteers, the average staff workload would go down. Staff need to be 18 or over so they can work past curfew...
Staff get benefits. Cheaper Reg (less than 1/2 price), Con T-Shirts at cost, the chance to attend the after con staff diner/party, etc. These aren't "free". You will be relied upon to help with the con. But, if you are like me, and find the actual con experience to be somewhat boring if you're not working, then staff is really what you should be looking at.
It's your call as to what you would like to do... but I highly encourage people to consider being staff. When the con is over, and people are all saying how much fun they had, that's when being staff really pays off.