Ah, the magic question that all of us ask at one point or another during our marathon training.
The short answer to this is whatever works best for one person might not work well for someone else. This is one area in which you need to experiment during your long runs to learn what you body enjoys, toelrates, and won't stand. Personally, I use a combination of things to keep me from getting bored eating the same stuff all the time. I use Clif Shots, Clif Bloks, and Sport Beans, and will generally have two packages of each with me during a marathon to eat myself and share with people who need it.
There are dozens of brands and flavors and formulas available, so it really does come down to a matter of personal preference.
John's right on with the nutrition piece. Go to the sports nutrition section of a large grocery store and the selection is vast.
Let me back up on the nutrition stuff for just a minute
First, the body burns fat and sugar (in the form of glycogen) as you run. Sugar is the hot burning kindling that you burn as you work at higher intensities. Fat is the longer burning logs that require a little starter fuel to ignite and then to continue the burn. Fat is the aerobic fuel you are training your body to use more effectively. Fat is abundant in the body and you could burn fat for a couple days if that were the only fuel source present (yes even a lean athlete has this kind of fat stored). Sugar on the other hand is in short supply generally less than 90 minutes if burned exclusively.
We train long and slow to adapt the body to a better fat burner but you will always tap a certain amount of sugar to keep the fire lit. Since sugar is in short supply we need to supplement the sugar stores as we go forward. If you do not you run the risk of hitting the wall the point where the brain says stop moving NOW as you are about to deplete my fuel
sugar is also what the brain uses as fuel.
A rule of thumb on fueling during activity is that you are generally ok with work up to about an 1:00 to 1:30. If going longer than that you should consider taking in sugar to keep blood glycogen levels from depleting. A great place to start is to fuel ( gel or other source if carbs) at about 45 minutes into a run. This is a great starting point but you may shorten that initial fuelling or lengthen depending on who you are (30-60 minutes). Then about every 30-45 minutes after that. This varies from person to person but these are good starting ranges.
There are several forms f fuel from gels, to beans, to blocks (think solid jello shots) to candy to real food. I would point you to more of a sport oriented product over a candy or real food. The sports oriented products will contain salts (electrolytes) to help replenish the sodium and other salts lost through sweat. Loose too many of these and you will cramp. Also, most sport product is packaged in easy to use containers that are designed to be used on the go. More serving sizes are in the 100-120 Kcal range. This is a critical size as if you go too much your GI system will react negatively. The gut can only process 200-250 Kcal per hour under load. Another strike against candy is that it tends to be sweetened with high fructose corn syrup. Its just not clear that this synthetic sweetener really delivers the cellular nutrition of a natural sugar.
For sport products look up Cliff, Powerbar, Gu, Hammer, Carboom, etc. Experiment with different forms, brands, flavors, etc. You will find quickly what works for your system and what does not. As you go through the years, you will also find that manufacturers play around with formations and what works today will not work next year. likewise, you may find that something working in the heat of the summer just does not cut it as the temps cool. Quite a few folks will find that chocolate flavors do not clear the throat well. You will see caffeine in several flavors. Experiment with caffeinated vs. non. For many caffeine will pass through.
Having run a few longer type events, I find that I start to hate gels (even when I have 2-3 flavors with me). For me it is because I will go for my favorite gel flavor first and leave the not so favorite for later. I find that if I randomly place gels know to work in a fanny pack (or in random order on my fuel belt) and not fish for the favorite I can go longer before I get to that point.
How to gel (eat) on the run. I will open my gel as I approach a water stop. Grabbing a water I take a swallow to moisten the mouth, gel and complete it, then wash it down with the remaining water. You will generally only get 2-4 oz of water in a cup. Practice with these volumes. When hot I will grab a second cup to assure that I clear my throat. Note this also applies to other forms of nutrition.
Cant take a whole gel? Follow the steps above but take only half the packet. Fold the top down so the gel does not leak out and keep it in your hand. Then as you approach the next water stop, complete that pack of gel.
Sport Drink. You should practice with PowerAde during your training. I find that if you can stomach pre-mixed drink you should be OK in the race. I still drink a cup of water with a sport drink to keep the concentration lower in the gut.
The general rule is to drink a sport drink when you hydrate in the race along with your water. However, you should avoid the sport drink at the water stops where you take in nutrition. That can and will overload the gut.
This is a lot of stuff and it will take some time for it to sink in. I will conclude with PRACTICE PRACTICE PRACTICE. The first time you practice, you may want to be in a tighter loop run where you will have easy access to facilities or home.
PowerBar and Gatorade have a great amount of data on their sites. Most of their data is not sales oriented though I would always read their claims with sales spin in mind.
I'm more of a slow and steady kind of gal. We're both really excited to do our first full marathon at Disney! 
Onward we go, one step at a time. 
