It's by a process of elimination that biscuits fit into the category of 'snacks'. They're too plain and simple to be served alone as an after-meal dessert and, because they offer few (if any) nutritional benefits, health and fitness fanatics don't eat them as meal substitutes or supplements in the way they might power bars. Biscuits are usually small enough (or should be, anyway - large ones just seem wrong) that they can be munched on as the occasional snack without evoking too much guilt about empty calories, or warnings that it will ruin your appetite for the upcoming meal.
The best biscuits are home-made. It's not just the benefit of knowing what has gone into them but also the heart-warming scent that fills the house as they bake. If you buy biscuits, check the label and go for those with the fewest multisyllabic ingredients.
All biscuits contain fat of some kind: it can be butter (for the best flavour); animal fat (such as lard or bacon dripping) for the lightest, crunchiest texture; and shortening, margarine or oil (for economy's sake). They also all contain some type of sweetener - usually sugar but it can also be honey, molasses, corn syrup or agave syrup (liquid sweeteners give a soft, bendy texture). Flour, whether from wheat (the most common) or another grain, binds the ingredients so they'll hold their shape when baked.
You can make biscuits seem more healthy by adding nuts, fruit and grains to a basic dough or batter, but don't fool yourself. Biscuits aren't unhealthy, unless you have pre-existing conditions that should make you avoid sweeteners, flour and fat, but they're not exactly nutritious (in any case, biscuits that are actually healthy don't taste that good). Biscuits, like many other snacks, should be eaten in moderation.