Can we inherit multiple classes?

No, multiple inheritances are not supported in .Net. 

What does the keyword "virtual" mean in the method definition?

Ans: The method can be over-ridden.

How method overriding different from overloading?

Overriding different from Overloading:

If we are overriding the method, derived class method behaviour is changed from the base class. In Overloading, method with same name by different signature is used.

OOP's Interview Question: What is OOP in C#?

Here i am discussing about, "OOP's related inter question's are asked up to fresher level".