For Red Wines:
Red wines are fermented using the entire grape, skins and all. When making red wines, two basic criteria need to be taken into account:
During fermentation, yeast create alcohol and CO2 gas. As the CO2 bubbles up through the wine, a portion gets trapped in the skins and causes them to float on the surface of the wine in a mass called "the cap". The cap actually lifts itself above the surface of the liquid and this causes the volume of must to expand in the tank. So, when filling red wine fermenters, keep in mind you will not be able to fill it to the top with must, you will need to leave enough space for the cap to expand into. Therefore, if you want to be able to ferment a set volume of red wine, you will need to size the fermenter slightly larger than the desired finished volume of wine.
The skins in the "cap" will need to be rinsed with the liquid wine several times a day during the fermentation. For large volumes this is done with a pump in a process called a "pump-over" (because you are pumping wine over the top of the cap). For smaller volumes (up to 300 gallons), this rinsing/wetting of the cap is usually done manually using a specialized "wine punch down tool" designed for the job. Using the tool, you simply break-up the cap and gently stir it back into the wine. This process is called "punching the cap". In order to use the punch down tool, however, you will need to have access to the surface of the wine. Therefore an open-top wine fermenter (not sealed) is the way to go for fermenting small red wine lots.
For White Wines (as well as Rose and Fruit Wines!):
White wines are made using just the juice from the grape. The crushing and separation of the juice from the solids happens before the fermentation begins. Since we have no solids present, there is no need to punch the cap. However, white wines are very sensitive to oxygen exposure and whatever we can do to limit this is a good thing. For this reason open-top fermenters are not recommended and sealed containers are the best choice for making white wines. If you are making small lots (1-6 gallons), then carboys are the ideal white wine fermentation vessel. However, for larger lots you will need to look at a Variable Volume or sealed tank.