It's mainly a matter of advertising. As vanilla is often rather bland (especially in cheap icecream), so much so that it has become a by-word for ordinary in many situations, "premium" flavours are sometimes called French vanilla to indicate that they will have a stronger-than-ordinary vanilla taste or smell.

From www.vanilla.com: "French vanilla is not a type of vanilla. It is a term used to describe an egg-custard base for ice cream. However, because it sounds exotic or romantic - especially in the perfume industry - it's often used to describe perfumes, candles and other specialty products." So it's marketing for some products, and for others it is a legitimate form of preperation.

According to one chef. . . "What is "french vanilla"? What is different about it from regular vanilla?" Chef Ottevanger answered: "The difference is lots more egg yolks." This makes sense when one considers that French Vanilla is actually "not a type of vanilla. It is a term used to describe an egg-custard base for ice cream." as stated in the answer given by ztorgo. Another explanation is given as. . . Flavor: Vanilla flavor in creams, cakes and other foodstuff may be achieved by adding some vanilla essence or by cooking vanilla beans in the liquid preparation. A stronger aroma may be attained if the beans are split in two; in this case, the innards of the beans, consisting of flavory tiny black grains, are mixed into the preparation. Good quality vanilla has a strong aromatic flavor, but foodstuffs with small amounts of low quality vanilla or artificial vanilla-like flavorings are far more common. By analogy, the term is used, often as "plain vanilla", in computing for default set up of a system, with no extras or modifications. Since "plain vanilla" ice creams are often almost tasteless, the term "French vanilla" is often used to designate preparations that actually have a strong vanilla aroma, and possibly contain vanilla grains. So, it is a matter of marketing to some extent, but there is a valid difference in content when it is applied to food preperation.

Throw some flecks of dried vanilla pods add marketing,increase the price and voilla!