A format proposed by Samantha Warren: color palette, heading and body type samples, and UI element samples like buttons, all on one page — with no actual screen layout. Where a moodboard uses a free collage to convey a mood, a style tile organizes real values (exact hex codes, font names and weights) into aligned sections to pin down "this is the style we're using."
It also differs from a mockup. A mockup renders one specific screen (say, the homepage) as a finished picture; a style tile is a set of materials that isn't tied to any screen at all. When you need to agree on tone before layout is even decided, it's faster to produce than a mockup.
Once approved, it becomes the reference other mockups and components are built against, and on larger teams it often grows directly into a design system's tokens and base styles.
When to use
Use it when tone (color, font, button style) needs agreement before layout does, or when it's still too early to draw a full mockup.