What is abstract class?

Answer 1

you can have methods defind but without content. If you extend it later you need to define it there.

Answer 2

abstract class means which contains partial implementation for methods.
abstract class A
{
abstract void m1();
void m2()
{
sop("m2 method");
}
}

conclusion: due to partial implementation next level child classes has to implement those methods by extending abstract class.
if a class is declared as abstract then you are not able to create object for that class either directly or indirectly.