Although generally associated with the Bordeaux region of France, Cabernet Franc's growth in Loire Valley predates that of Bordeaux. Along with Sauvignon Blanc, it's one of the parent strains to Cabernet Sauvignon. By the mid 18th century though, Cabernet Franc had become firmly ensconced in Bordeaux and is one of the 3 largest varietals grown there (behind only Cabernet Sauvignon and Merlot). It's the dominant red wine grape of the Loire Valley. It's also used to soften and round out many Cabernet Sauvignons offerings.
Because of its close association with Bordeaux style wines, it 'tagged along' to many of the newer areas that embraced Cabernet Sauvignon. Over the last 20 years, its attracted its own following and is now a favorite for many who enjoy its slightly softer, mellower side when compared to Cabernet Sauvignon. Despite assertions in the movie Sideways that it has no character, a well made Cabernet Franc is a well balanced blend or fruit, tannins and acid.
Vines are vigorous and generally ripen sooner than Cab Sauvignon. It's prone to overproducing fruit leading to problems in fully ripening so careful cultivation and pruning is necessary to insure quality fruit. Cab Franc will tolerate cooler temperatures than other wines grown in Bordeaux which explains its popularity in the Loire region, which is farther north than Bordeaux.
Cabernet Franc grapes are smaller and thinner skinned than Cabernet Sauvignon producing less tannins and contain less acid which leads to its mellower flavor. The flavor of the grape is often described as vegetative or herbaceous, especially when not fully ripened.
Cabernet Franc poses more challenges to winemakers than other heavier reds, which can more easily hide imperfections. Unripe or over production of Cabernet Franc fruit can cause the herbaceous flavor mentioned above. Over ripening can increase the tannins in the skins but without the structure of the heavier wines, they can overwhelm the wine. If everything comes together however, the final product can be an exceptionally well-balanced, medium body red that will age extremely well.