Interface vs. Virtual vs. Abstract vs. Inheritance

abstract class TwoDShape {
 variables
 methods
 ...
}

public interface ISeries {
  variables
  methods
  ...
}

class MyClass {
  variables
  methods
  ...
}

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It seems the syntax in the main code when it comes to defining a class which {inherits a regular class/implements an interface/inherits an abstract class} is the same for all three, is that correct?
Then in main code:

class Triangle : TwoDShape {
}
class ByTwos : ISeries {
}
class MyExt : MyClass {
}

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Is the virtual keyword only for methods and variables and not for classes?

Interface - contract "You must implement these methods." Contains no code.
Abstract - class meant to be inherited. Class can not be instantiated. Contains code.

What is the syntax for a class which implements more than one interface?

Virtual - keyword placed on methods "Here's a default method to use unless you override it with something better."

So do I have this correct so far?