Saw this linked in another post, felt the urge to type something up ^^. It's just a half-draft crap shoot, but maybe someone can use it.
A unit is a specific player or NPC. This includes your character, your pet, your party and raid members, their pets, targets, your target's target, and your focus. Grid tracks all members of your party or raid, including yourself. Frame
A frame is a container object in the WoW UI system. They are the fundamental building blocks of a graphic interface and are used to separate functions and information. Grid itself is a frame, containing smaller frames for each unit it is tracking. Status
Status refers to a unit's condition. This includes it's HP, mana, buffs, debuffs, and location. Grid is designed to display as much of a unit's status as possible in a small and configurable manner. Indicator
An indicator is a visual display of a unit's status. Examples include health bars, (de)buff icons, and text. Layout
A layout is a collection of indicator and frame settings. You can have multiple layouts for a character in grid (for instance, healing and buffing), and you can have layouts for different characters (for instance a mage decursing layout and a priest healing layout). Aura
An aura is a buff or debuff. Don't confuse this with paladin auras, which are auras in this sense, but not the only ones. Any buff or debuff can be called an aura.
Forgive me if it's too complex or error ridden. I figure anything's more informative than "edit me" :P