I heard people use this term many time but I am still not quite sure what exactly a flag is in c, is it like a boolean value? Can someone give me good explaination or direct me to some websites that do? thanks.

If you are inside the C code, then yes, it's generally a boolean concept, implemented as an int. However, it's possible for a flag to have as many values as you need...
Some people do also apply the term to a cmd line option that takes no value ie it's either specified or not.

A flag is nothing but a status indicator. Any variable can be used as a flag. Boolean vars are usually used for flags since they take only one bit space and can be used to check an 'ON/OFF' condition.
Like for instance, you add a couple of numbers, and if the result is negative, then you can set a flag to 1. If the sum is positive, set it to 0. Now, elsewhere you can check the status of this addition by looking at the value of the flag var. <Elementary concept, used very often>

<gave you this eg, since there is a signed bit flag [register] in all processors which does the above operation.>

suresh

To the above I'd add that you can also have an int, used for a bunch of flags, using the bitwise operators. see Bitwise manipulations.