This time of year wineries all over the northern hemisphere are taking delivery of freshly coopered oak wine barrels. "Barrel" is an almost uniquely American colloquialism that is occasionally sneered at outside of American winemaking circles. What we think of as wine barrels is usually known in the rest of the world by the French term, barrique. However, barrique refers specifically to the Bordeaux style of wine barrel that holds 225 liters of wine (or 59.4 US gallons). The Burgundy style wine barrel holds 228 liters (60.2 US gallons) and is slightly shorter and fatter around its midsection. When referring to any size oak wine barrel, the term is usually cask. Wine barrels are typically the second most expensive non-overhead input in the production cost of a bottle of wine (the first is grapes itself). Wineries therefore experiment with larger format barrels to reduce this component cost. If you construct a wine barrel with the length of a barrique and the circumference of a pièce (Burgundy style barrel) you get a 265 liter barrel. These are popular in the USA because they fit on barrel racks constructed for their smaller cousins, but hold about 17% more wine. Other popular formats of wine barrels in the USA include puncheons that hold 300, 400, or 500 liters. Funny enough, a 300 liter barrel is sometimes refered to by its old English name, hogshead.
Taking delivery of new barrels is one of the singular pleasures for a winemaker. In a production environment where everything is always in constant state of cleaning, new barrels tantalize the senses. Oak is largely neutral in character but the toasting process produces a sweet, fresh, slightly toasted aroma with subtle vanilla and coconut character. They're theoretically sterile on the inside (sanitation is constant concern in a winery), and they look good - they're the only barrels in a winery that aren't stained. Even water stains barrels.
In winemaking, barrels serve two functions. They are "cooperage," meaning they act as long-term storage for wine in a state in which the vessel is completely full. While it's commonly thought that wine tanks serve this purpose, wine tanks can be rather poor vessels for long term storage. This is because wine tanks are often designed for other purposes (fermentation, bottling, blending, etc) and unless the tank can be kept completely full without pockets of air at the top, the wine will spoil rather fast. In a small winery the top of the tank can be difficult to access if it's serviced only by ladders. Red wine fermenters, for example, are the worst tanks for long term storage. Their design typically encourages the largest surface area of wine for the volume they hold, and access at the top of the tank is usually through very large doors (lids on a hinge) with large surface areas even when completely full. Barrels, however, are easy to completely fill, the exposed surface area of wine as you fill the barrel diminishes to almost nothing, and are typically worked with on the ground - no reaching, climbing, or avoiding because of difficulty of access.
Of course oak isn't required to accomplish the goal of long term storage. 55 gallon stainless steel drums will suffice at a fraction of the cost of new oak barrels, and will essentially last forever. Oak's second function in winemaking is in its additive value. With this it becomes a part of the winemaking process, not just a convenient vessel to hold wine. While oak's impact and importance in the winemaking process is taken for granted by both winemakers and customers alike, explaining exactly how it impacts the winemaking process is much more difficult. Its most conspicuous impact is to flavor the wine. While oak is rather neutral in character, the coopering process creates chemicals not present in raw oak. The character and abundance of these chemicals is unique to each cooper. Many coopers share the same sources of raw oak, so differences in barrel character result from processing beyond the raw oak stage. This includes mainly seasoning - the process by which the oak staves are dried outside and subject to minor chemical alteration by the impact of microorganisms, and toasting. Originally used as a way to bend the staves into the shape of a barrel, toasting is now critical in how it impacts the character of a wine. This is a relatively new concept in barrel cooperage. In whiskey barrel production, barrels are charred and there is much more homogeneity in the character imparted to whiskey across all coopers. In wine barrel toasting, the interior of the barrel varies from a mild tan/brown color to almost black (but is in no way charred). This is a function of temperature. But toasting is also impacted by the length of time at a temperature, how fast the temperature is ramped up and down during the process, and whether the heat is supplied by fire or heating element.
The specific flavor/aroma impact of oak barrels on wine can be difficult to characterize. Where obvious toasted oak character is sought, barrels can impart vanilla, "whiskey," smoke, and coconut character. The choice of French or American oak will favor some of these characteristics over others. In a very broad sense, French oak offers more complex, spicy aromatics whereas American oak veers towards a more rustic "oaky" characters with plenty of vanilla and some coconut. But I've also encountered rather sophisticated American oak barrel character as well as clunky, overly perfumy French oak. But a discussion of barrel aromas just scratches the surface of what is really sought after in a barrel program. Most conscientious winemakers don't pursue an abundance of obvious oak character in wine. A rule of thumb that many of us follow is that the proper amount of oak is as much as the wine can handle and barely be perceptible (if at all). In some wines (e.g. Sauvignon Blanc) this may be next to nothing and with others (Cabernet Sauvignon) it could be a program of 100% new barrels.
Perhaps more important than obvious toasted oak character in wine is the other aspect of barrel character - structure and texture. Oak is a source of tannin that imparts a focused, somewhat rigid structure to wine. It's also a source of sweetness, not sugary sweetness, but a richness and roundness that is often preferred to sugar sweetness. To what extent oak contributes these characters to wine is a function of oak species, where the tree was grown, and how the cooper seasoned and toasted the oak. Some of my favorite coopers for red wine in the Mid-Atlantic contribute practically no obvious oak flavor/aroma but plenty of structure that helps to focus the character of the wine. This type of oak is also beneficial for delicate white wine varieties like Sauvignon Blanc that are easily overcome by obvious oak character. On the other hand, some wines (e.g. Chardonnay) seem to benefit more from oak sweetness than from barrels that offer tannin and structure.
The best way to use barrels in the winemaking process is a very personal stylistic decision that often has a very large range of acceptability. Determining what works best is a lifelong pursuit that changes with respect to variety, grape growing region, and site specific vineyard characteristics. Winemakers are often always experimenting with new coopers and new offerings from the same coopers with respect to oak source, seasoning, and toasting. Some of my favorite barrels are the traditional favorites in the industry while others have been accidental finds. I've encountered truly awful French oak barrels and stunning American oak, and everything in between. But discovering what works well for the grape varieties you work with at your specific site is one of the joys of making wine.