Oxidation is the degradation or modification of beer when it comes in contact with oxygen or an oxidizing agent. Oxidization in beer is inevitable and occurs in beer at all times regardless of how it is stored. That being said, there are ways to diminish the rate of beer oxidation.
A good rule of thumb is to prevent oxygen from coming in contact with your beer whenever possible. The only time that oxygen should intentionally be introduced to a beer is when aerating or oxygenating wort prior to pitching yeast, and, in rare cases, to facilitate a souring process. Great caution should be taken when transferring, racking, kegging, and bottling beer to avoid splashing it or exposing it to more oxygen then is necessary.
When possible, fermentation vessels, kegs and bottles should be purged with CO2 prior to and after having beer transferred into them. Heat is another factor that impacts oxidation, and, whenever possible, beer should be stored cold to reduce oxidation rates and to preserve its freshness. Another way to help slow the impact of oxidation is to keep bottles and fermenters shielded from sunlight.
Beer that has been oxidized typically has a stale or paper-like taste. In darker beers, you may also smell or taste sherry notes.