Generally, the parameters are what are used inside the function and the arguments are the values passed when the function is called. (Unless you take the opposite view — Wikipedia mentions alternative conventions when discussing parameters and arguments).
double sqrt(double x)
{
...
return x;
}
void other(void)
{
double two = sqrt(2.0);
}
Under my thesis, x is the parameter to sqrt() and 2.0 is the argument.
The terms are often used at least somewhat interchangeably.