A dose you can count
A tablet or capsule is the simplest promise a medicine can make: one object, one dose. This is the idea of the unit dose — the dose is fixed at the factory, not measured by the patient with a spoon. That single fact explains most of why solids dominate. A liquid relies on the person pouring it correctly; a tablet carries its active ingredient in a sealed, countable package that travels in a pocket and survives a year on a shelf.
Solids are also chemically calm. With little or no water around, the reactions that spoil liquids — hydrolysis above all — slow to a crawl, so a dry dosage form keeps its label claim for longer. And they are cheap to make at huge scale: a single tablet press can stamp out hundreds of thousands of tablets an hour.
Two families: pressed and filled
A tablet is powder compressed under tonnes of force into a solid disc. A capsule is powder (or granules, or even a liquid) enclosed in a shell — most often the two telescoping halves of a hard gelatin capsule. Pressed versus filled is the great fork in the road of solid dosage forms.
Almost nothing in either is pure drug. The active ingredient might be a few milligrams; the rest is excipients — the inactive helpers that give the dose its bulk, hold it together, make it dissolve, and let it run through machinery. A good formulation is mostly the art of choosing those helpers, which is what the rest of this track is about.