OG stands for Original Gravity –
This refers to the ‘original’ or starting gravity of the wort before fermentation commences. This along with the FG are essential in allowing you to calculate the approximate % ABV of your beer.
FG stands for Final Gravity –
This refers to the final, end, or finishing gravity once fermentation is complete. It is also sometimes referred to as the terminal gravity. This value is essential not only in allowing you to calculate approximate ABV, but also to allow you to determine whether fermentation has completed as expected.
SG stands for Specific Gravity –
This has two meanings; it is a general term for ‘gravity’ (or density) of the liquid relative to water; in this sense both OG and FG are also types of SG, ie original specific gravity or final specific gravity. However, it is also commonly used as an abbreviation for ‘current’ specific gravity, which means the gravity at any given time, often referring to any gravity readings taken during fermentation.