Answer 1

Any variable used to see the state of some operation during execution can be called a flag.

It is used in wide range of applications, such as marking some operation as done in a loop (Suppose you need to do some extra calculation only when some condition is true, for a series of operations, based on the previous result of the operations), or marking the state of some operation (like a button, was it pressed or not?).

A simple example in C++: To check for prime :

int no; cin>>no;
bool flag=0;
//My flag, to mark if the number is divisible by something or not.
for(int i=0;i<sqrt(no);i++)
{
if(n%i==0) flag = 1;
//Something divided my number.
//The flag is set to one as a result.
else continue;
// Continue to next iteration.
}
if(flag) //If the flag was true?
cout<<no<<"is composite"<<endl;
else
cout<<no<<"is prime"<<endl;

Thats it. A flag just helps mark a state.

An advanced use will be in a button, where you need to roll some changes on screen only when the button is pressed in a separate message box, dialog box, or window.