This is a very deep subject. Too deep to be answered by a post, or by a thread. I suggest going to a site dedicated to wine, poking around the forums and asking your questions there.
Cabernet Sauvignon and Merlot are different wine varietal species of grapes. They have different taste and aging characteristics. What the vintner does on the way to getting them into the bottle depends on the vintner. Although I hate to disagree, most good Merlots are, in fact, aged in oak.
Cabernets are known for their ability to handle long periods in oak, i.e., barrel aging. Typical wine tasting cliches (true cliches) regarding Cabs are "pepper," "eucalyptus," "raspberry," "mint," "chocolate," and "raisin." (The last from an intentional, slight over-ripening.) Cab blends well with other grapes and, indeed, is often blended with Merlot. Cab, Merlot and Franc is the basic Bordeaux blend.
Merlot usually produces a softer, fruitier and less complex wine than Cab. It's reasonable to say that pure Merlot doesn't have quite the affinity for oak that a Cab does. Most of the wine tasting cliches related to Merlot refer to its fruit notes. When Merlot is blended with other grapes, it is seldom the dominant note in the blend. Merlot is currently very trendy, over-produced and made into wines not really suitable to the grape's character -- much as happened to Chardonnay in the previous decade. I'd be careful spending too much on age. Merlot really doesn't have the legs.
Sherry is a fortified wine made in an area of Spain called Jerez de la Frontera. The word "sherry" is a corruption of "Jerez." Indeed the wine is referred to as Jerez in the non-English speaking world. Sherry is made in a special type of winery called a solera. There the young wine is mixed with a little brandy to make it stronger, then progressively aged and blended. Some varieties are sweetened. In America we usually think of sherry as sweet. But in fact, most sherries are rather dry. The three driest varieties from sweetest to driest are Amontillado, Manzanilla and Fino. Again, at risk of seeming disagreeable, these are appropriate with food. All three with appetizers and the second two with any number of foods -- if you are of a mind. FWIW, when drinking as an aperitif, the drier sherries are best with a little chill on them.
A good sherry is a wonderful thing. A great sherry is a revelation. .
BDL