Gingivitis is a disorder characterized by swollen, recessed gums. It is caused by long term plaque buildups at the base of the teeth. Bacteria are attracted to these deposits of plaque and break them down, but also produce toxins that inflame and infect the gums below.
Symptoms of gingivitis include sores on the gums, swelling of the gums, a bright red appearance of the gums, shininess of the gums, bleeding gums when brushing gently, and tenderness. If you experience any of these symptoms, you might want to mention it to to your dentist even though they will be able to tell if you have gingivitis anyways. It is always best to catch the disease in its early stages so you can easily reverse it. While you can probably cure gingivitis with diligence in practicing good oral hygiene, you will not be able to reverse any tooth decay that.
The best way to prevent yourself from getting gingivitis is to brush, floss, and wash your mouth. Brushing your teeth removes the plaque deposits that feed bacteria and without any food, the bacteria won't be collecting and producing toxins. Flossing helps to remove the plaque that gets between your teeth, where the average tooth brush has difficulty reaching. The spaces between the teeth are accessible by bacteria and should not be overlooked. Lastly, rinsing your mouth with an alcohol-based mouthwash (or just drinking some alcohol) will kill the bacteria in your mouth. Obviously, without any bacteria, there won't be any toxins to infect and inflame your gums! All of these methods are very important in maintaining excellent oral hygiene.
It should also be noted that excessive oral care can contribute to gingivitis. If you are brushing your teeth so hard or so long that it is damaging your gums, you might be overdoing it. Two minutes of brushing is the general rule and anything more probably isn't going to help very much. While overbrushing the teeth won't hurt, overbrushing the gums will since it creates portals for infection.
If gingivitis is allowed to progress without any intervention, it will severely weaken the support structures of the teeth. You may find that your teeth become flexible and move under pressure. This is definitely not good and will make chewing difficult and painful. If the gums become sufficiently weak, your teeth will begin falling out and force you to get prosthetic replacements.