Differential Calculus deals with finding out how Values of Functions Change and the Rate at which they Change when the Variables that those Functions are dependent on themselves change.
For Any Variable, the term Differential means Infinitesimal Change in the Variable (i.e. the change that tends to zero). For example, if a variable is denoted by \(x\) then an Infinitesimal Change in \(x\) is known as Differential of \(x\).
For Any Function dependent on One or More Variables, the Derivative of a Function with respect to a Particular Variable is the Instantaneous Rate at which the Value of Function changes when the Variable Changes Infinitesimally.
Also for Any Function, the term Differential means Infinitesimal Change in the Value of Function when the Variables that it depends on Change Infinitesimally.
The process of calculating Derivatives and/or Differentials of Functions is called Differentiation.