I Am In Class 9Can you explain me differentiation?i came across many questions that use differentiation.

differentiation in class 9 cannot be applied in maths..its application will only be in physics.
as per to physics.the derivative or rate of increase or decrease is differentaition.
for a func. f(y)
d/dx f(x)=f''(x)
der. of x^n=n.x^n-1
der. of sinx=cosx
der. of cosx=-sinx
thats all req. at class 9
for depth please follow class 11 book

to tell difference between two or more thing

yes akash is also right 

differentiation have a link with derevative

but it depend on you which def..

you need