Shell

A shell is your outer layer.

Waterproofs are also known as Hard Shell. The best waterproofs are not only waterproof, they also "breath" (ie let your sweat out) to some extent. Anyone who has had the misfortune to use a non-breathable waterproof will realise how effective modern breathable waterproofs are, but unfortunately even the best, as they rely on a gradient of higher humidity on the inside passing to lower humidity on the outside, do not tend to work as well when wet (which is of course when you need them to work most).

Apart from rain trickling in the hood and sleeves, the major source of moisture is your own sweat. The ability to disperse this sweat is known as "breathability". In general, the more expensive waterproofs tend to be the most breathable - however even these have their limit and that limit is currently well short of what a sweaty body carrying a large rucksack up a steep hill can produce.

The result of all this is that they are not as breathable as they really need to be and you will be better off trying to avoid wearing them as much as possible.

The remaining categories are not waterproof but in many situations will be better than waterproofs. This is because even the most breathable waterproofs are not breathable enough for active use, so, while it's important to have a waterproof shell in your rucksack, it's best to wear it as little as possible.

Windproofs are lightweight highly breathable outer shells with little or no insulation. They will not prevent rain getting in but do prevent sweat build up by allowing your body to breath.

Soft Shells are heavier duty than windproofs, are still more breathable than waterproofs and are usually shower proof. They are harder wearing but heavier and more expensive than windproofs.

Buffalo is an unusual system which combines all the layers into one and does away with the need for waterproofs. It is best suited to sub-zero conditions.

Windproofs

Lightweight outer layers designed for maximum breathability with a high level of windproofness but little rain protection.

Soft Shell

Midtweight and heavy duty outer layers designed for very good breathability with a high level of windproofness and some rain protection.

Buffalo

The Buffalo Double P System was softshell long before anyone had coined the term. A unique way of keeping warm and comfortable in sub-zero temperatures without using waterproofs or underwear.

Waterproofs

Waterproof and windproof outer layers designed with oodles of breathability (if you read the manufacturer's blurb) and limited breathability if you talk to anyone who's used one.