Euler:
Pronounced "oiler" Euler is a common method for calculating 3d rotations. Is popular because it uses 3 values X, Y Z angles (360 degree equals one full rotation). Euler requires a "rotation order" and can experience gimbal problems, the worst resulting in Gimbal Lock when 2 axes align. This is why it's sometimes refered to as Gimbal, instead of Euler.
Gimbal:
Gimbal is the effect caused when Euler rotation axes break the usual 90 degree angle difference on a rotation manipulator. The extreme of this is gimbal lock when two axes align completely.
Key or Keyframe:
Setting a value and recording it on a time value (frame) in animation.


Manipulator:
The widgit/tool that appears to help us visually click and rotate/move/ modify an object or selection.
World/Local Coords:
Rotation manipulators usually default to world or local (object) modes to rotate objects or components (verts edge faces). In local or object mode the manipulator will rotate with the object or section. In "world" the rotate manipulator stays aligned to the xyz layout of the scene.
Rotation Order:
The order (x, y and z) that the computer calculates the rotation of an object when calculating using Euler Rotations. There are 6 possible combinations.
xyz, xzy, yzx, yxz, zyx, zxy
Gimbal lock will occur when the middle rotation axis is rotated 90 degrees from zero.
