Steel, silver and crossbows in The Witcher 3
Steel swords in The Witcher 3 are for humans and most beasts, silver swords are for monsters, and the split is absolute enough that carrying the wrong one into a fight is the most common reason damage looks broken. Crossbows sit apart: they do little damage but stagger, and underwater they are the only weapon that works.
Each Witcher 3 weapon lists its damage range, the bonuses the game files attach to it, the number of rune slots it carries and the level it expects.
Witcher 3 swords that are crafted, looted or bought
The strongest swords in The Witcher 3 are crafted, and the witcher school lines on the gear sets page run in tiers that each need the previous diagram. Relic swords found in the world can outclass an early tier, which makes them worth keeping until the next ladder step is affordable.
Rune slots are what separates two swords with similar damage. Filling them is covered on the Witcher 3 runes and glyphs page.