I usually use a hop bag when I make beer. A hop bag is a nylon bag that keeps the hops contained and easily removed from the wort before fermentation. This helps keep the beer a bit cleaner. In addition off flavors can be leached from the hop residue as it sits on the bottom of the fermenter. Removing the hop residue before fermentation helps prevent this.
The main problem with using a hop bag is getting hops into the bag while it is in the kettle. The boiling wort (3 gallons of it) is hot. It's boiling. Boiling water is hot. The bag is full of liquid and cannot easily be removed from the boiling wort. Opening the bag is tricky because the wort is hot (remember).
So, I came up with my own solution: a funnel. I stick a large funnel into the opening of the bag and slowly add the hop pellets. The pellets must be added slowly so they don't plug the funnel. This method also only works for pellets. On occasion, I have hydrated the pellets in water and poured the slurry through the funnel. That works too, but is slower.
Here's a picture.
 Brief hop background: There are two main uses of hops: bittering and aroma. For bittering to occur, the hops must be boiled for an hour during which chemical isomerization occurs. For aroma, the hops are typically added during the last 15 minutes of a 60 minute boil.