Difference between Rigidbody2D and rigidbody2D

Questions & Answers
1

Every component in script has Rigidbody2D and rigidbody2D, Rigidbody and rigidbody, Animation and animation, Collider and collider,etc.

What is the difference between them(Uppercase and lowercase)?

2

Capital letters are the class. Lowercase letters are (pre-Unity 5) shortcuts to the instance of that class attached to the current object. i.e.

X.transform

is shorthand for

X.GetComponent<Transform>()
3

Uppercase is the class, the type, while the lower case is the reference variable.

Transform transform;

is the reference to a Transform component and the reference is called transform.

They could have called it:

Transform robSchneider;

but then it is totally irrelevant.