Gels are little packets of a flavored gel type substance that people take to give them energy when exercising aerobically for long periods of time. They claim to be better than eating regular power bars, granola bars, or any other type of solid because their gel form allows the carbs/glycogen to absorb quicker into the body, and they're lighter on the stomach. The directions on the packets vary, but they basically say to consume them every 45 minutes or so, followed by water, to maintain the energy needed to complete your goal. Here are some website of popular brands:
http://www.gusports.com/html/gu_energy_gel.htm
http://www.prolithic.com/hpages/efoods/powergel.html
Supposedly they have the same effect as eating a banana, or some honey, they're just more convinient. Basically they're just a quick carb fix.
I'm not exactly sure how glycogen works, or how it's even related to this, but from what I've read elsewhere there are two different opinions on how quickly it depletes and if it's necesarry to replace. Some, like danny_boy say that it depletes in 45-60 minutes, thus needing the replacement in any exercise over this period of time. Others have said that you don't really need it for runs under 2 hours or so, because the glycogen isn't nearly depleted by that time.
Anyway, I went ahead and tried them in my long run today, and I'm not exactly sure how much of an effect they had. I seemed to have more energy, but that could be due to the fact that I was drinking more water than I usually do, or maybe it was just a good day. They don't seem to be doing any harm, so I'll keep using them just incase they are giving me some benefit, but it'd be nice to know if they're actually necesarry.