What is abstract class ?

abstract classes can contain methods that don't require an override and methods that do require an override,  since it can contain methods that do require and override it must be extended to be constructed.  useful if you want to share code among several closely related classes that impliment a common interface

eg:
public abstract class abscl {
public void m1(){ // Defined and Implemented
System.debug(' I am in abscl: m1');
}
Public void m2(){ // Defined not implemented
}
public abstract void m3(); // Declared: Not Defined and Not Implemented this should be declared abtract method not implemented body method.
}

child class
public class abschd extends abscl{
public override void m3(){         // the abstract method should be declared in child class using override but object is created in child class only.

System.debug('I am in absch: m3 ');
}
}

Thanks