Primary Fermentation Buckets
Normally oxygen is the enemy of wine, because it causes the wine to taste raisin-like and flat. The wine doesn't really go bad, in the sense that it's spoiled or dangerous to drink, it just looses its appeal.
The one time in the life of a wine where this is the exception is during the primary fermentation. Primary fermentation begins as soon as the yeast is added to the "must", which is the slurry of grape juice and skins that you have after the grapes are crushed. During this initial period, the yeast (actually microscopic plants) need oxygen to get going, and after the yeast start working they will generate carbon dioxide gas, and this gas will blanket your must and protect it from oxygen.
Important Primary Fermentation Bucket Characteristics
So, the purpose of your primary fermentation bucket is to keep your must contained during the fermentation, and to keep fruit flies (that can carry vinegar-making bacteria) and other foreign matter (like your cat) out of your wine. You can buy a primary fermentation bucket either online or at your homebrew store, and I advise that rather than trying to use a plastic garbage bucket or some other container that you might not be sure is of food-grade materials.
It is important that your primary fermentation buck have a lid, that is it big enough to contain the volume of crushed grapes for the amount of wine that you want to make in one batch, and that it is convienent to move around and to clean. For a 5-gallon batch of wine, starting with about 70 lbs of wine grapes, you want at least a 10-gallon container, although this will be filled to near the top with the grape skin and juice slurry. A container one size larger can be more managable (if you have a few more grapes than anticipated), although a 10-gallon bucket is generally a good choice.
I find that I like working best with a simple plastic primary fermenter with a lid that fastens securely but is not fitted with a gasket to be airtight. The lid on these fermenters is easy to get on an off, they have convienent handles, and they are light to lift and easy to clean. One source online is from morebeer.com.
Some primary fermenters come with airtight seals and rubberized o-ring holes in the top where an airlock can be fitted, but unless you are going to attempt what's called an extended maceration (letting the grape skins marinate in the grape juice for an extended period after the primary fermentation completes) that's not really necessary. I don't advise trying an extended maceration, by the way, until you are very experienced and have read up on the topic (even then it's a gamble). You would also want the airtight container to attempt a Beaujolais-style wine with a process called carbonic maceration, but that is a very advanced technique as well.
I have found some of the less expensive containers with airtight seals to have lids that are very difficult to remove. In fact, I have one such container that I no longer use because prying off the top actually left my fingers sore. I have another container with a wonderful spin-seal lid that makes an effortless airtight seal, and it is a nice 14-gallon size, but its size and box-style shape (instead of the traditional narrowing cylinder shape) makes it difficult to get into the basement sink for cleaning.
Some primary fermentation containers come with a tap at the bottom, with the thought that you could drain off the wine after fermentation. My advice is to avoid these. After the primary fermentation, there will be a thick layer of sediment on the bottom of the fermenter, that will immediately clog this tap. For the winemaker, this tap ends up just being a difficult area to clean.
Alternatives to Buckets
Some winemakers have been known to use ceramic crocks, such as those for making pickles, for a primary fermenter for small quantities of wine, covering them with plastic wrap secured with a rubber-band and poking some holes in the top with a knife to let the carbon dioxide out. But, crocks are heavy and easily broken, and plastic primary fermenters are not so expensive, so I really recommend getting something that was intended for the job.Next: Airlocks