The distinction is the reason the vocabulary is so confusing from outside. One factory size can appear under a dozen different commercial names across the brands it produces for, and a single commercial name can cover materially different cigars at different houses.
The names a reader will meet
Robusto, Corona, Churchill, Toro, Lonsdale, Lancero, Panetela. None of them is standardised across the industry — one house's Toro is another house's Gran Corona — so a size name is a strong hint and never a measurement.
Parejo and figurado
A parejo is straight-sided with a rounded head, which covers most cigars sold. A figurado is any shaped cigar: tapered at the head (torpedo, belicoso), tapered at both ends (perfecto), or bulging in the middle (diadema). Figurados are harder to roll consistently, which is most of why they cost more.