I'd like to add that the tasks and mental work that are expected of you will differ depending on the unit where you are hired. Anything that isn't in your training, you'll be oriented to "on the job". Some wards in our hospital, the NA's don't empty colostomy pouches, some they do (for example). I work on a trauma unit, and I recently found out that "logrolling" technique is NOT part of the NA course here... we have to teach all the new ones on the job about how to do it and the importance, etc.
Also some wards, you'll get more of an atmosphere of teamwork than others. Some places you'd literally be treated like a menial servant to do the tasks that nurses feel are beneath them. (Not on my unit - I was an NA before, and thats part of why I chose to work someplace else as a nurse, I like teamwork.) Your best bet is to shop around, talk to other CNA's in your area and find a work environment that suits you.
What you'll be allowed to do, and what you'll be responsible for - that is a product of your state's nurse practice act (dictates what you have to be an RN or LPN in order to do, so some things can't be delegated to CNA's), plus your hospital's policies.
NA's in my hospital can't do vitals or blood sugars. They do personal care and hygeine, toiletting, they help pt's mobilize as advised by the nurse (assisted walks in the hall, etc). They assist the nurses and physios with activity and mobilization. They help us get pt's on and off stretchers to go for tests. Change bed linens. They are responsible to mark down intake and output. They deliver trays, and measure what pt has eaten or had to drink. They empty and measure foley catheters, urinals and hats. Turning and changing pt's every 2 or 3 hours as needed. Plus other things that may come up, as long as it doesn't require a nurse. (Night shift NA's do a lot of restocking as well).
My pet peeve is NA's forgetting to mark down a pt's urine output, and I have to ask them later, hoping they remember. On the whole, I love our NA's though... they are terrific with the pt's and it's a pleasure to work in a team with them.