Outline
Stroke icons that follow the text color. Lucide, Tabler, and Carbon are outline sets. Heroicons, Phosphor, Solar, and Remix keep an outline variant beside their other drawings.
Styles
Outline, filled, duotone, and multicolor. This is how an icon is drawn, separate from what the set is for and from a word in the name.
Filled
Solid shapes. Material Design Icons are filled. Heroicons calls the same idea solid. Tabler, Phosphor, Remix, and Bootstrap add a filled variant next to the outline.
Duotone
Two tones in one drawing. Phosphor and Solar ship a duotone variant for the same names.
Multicolor
Icons that keep their own palette: flags, emoji, flat color illustrations, file icons, and most logos. They do not follow currentColor.