Specific gravity (SG) is the density of a liquid measured relative to plain water, which is defined as exactly 1.000. Wort and beer read above that baseline because dissolved sugar is denser than water — the more sugar dissolved, the higher the reading, which is the entire mechanism behind using a hydrometer or refractometer to track fermentation.
The single scale underneath OG, FG, and attenuation
Original gravity, final gravity, and apparent attenuation are really just specific gravity read at different points in the process and compared against each other — there's no separate measurement system involved, just the same SG scale applied before and after fermentation. Understanding that specific gravity is the common thread makes the relationship between all of this site's gravity-related terms much easier to hold in your head at once.
Other scales exist and read the same underlying property differently
Plato and Brix both express the same basic dissolved-extract property specific gravity does, just on different numeric scales more common in continental European and winemaking contexts respectively — a wort read as 1.050 SG corresponds to roughly 12.4°P, the same physical property described two different ways. Homebrew recipes and calculators on this site work in specific gravity throughout, so a Plato or Brix figure from another source needs converting before it's directly comparable.