This isn't a medical definition, but it's basically an area where your body has trapped pus and prevents the pus from escaping. This irritates the region and results in a painful spot. It is kind of like a pimple, but much larger. What they'll generally do is cut into the abscess to drain it and then leave it open. You'll likely be asked to "pack" it with gauze for a 3-6 weeks (2-3 times a day). This holds area open and allows the body to heal from the inside out. It'll probably leave a scar and if you're like me, you'll feel like this is a midevil practice. Fortunately, although painful, it is very straight forward and should heal just fine with the process. Cleaning and packing isn't fun and is painful for the first 3-7 days, but then you'll get used to it and it won't be a big deal. It's not awesome to have, but it is far from the worst complication. I had three abscesses because my body reacted to the stiching material used. I had to "pack" for about 14 weeks total and consider myself the Michael Jordan of packing.
Hope this helps!
Here's the Wikipedia definition:
An abscess (Latin: abscessus) is a collection of pus (dead neutrophils) that has accumulated in a cavity formed by the tissue on the basis of an infectious process (usually caused by bacteria or parasites) or other foreign materials (e.g. splinters, bullet wounds, or injecting needles). It is a defensive reaction of the tissue to prevent the spread of infectious materials to other parts of the body.
The organisms or foreign materials kill the local cells, resulting in the release of toxins. The toxins trigger an inflammatory response, which draws large numbers of white blood cells to the area and increases the regional blood flow.
The final structure of the abscess is an abscess wall, or capsule, that is formed by the adjacent healthy cells in an attempt to keep the pus from infecting neighboring structures. However, such encapsulation tends to prevent immune cells from attacking bacteria in the pus, or from reaching the causative organism or foreign object.
