The AGS scripting engine supports the following operators in expressions. They are listed in order of precedence, with the most tightly bound at the top of the list.

WARNING: When using operators of equal precedence, AGS by default evaluates them right-to-left. So, the expression a = 5 - 4 - 2; evaluates as a = 5 - (4 - 2); which is not what you might expect. Always use parenthesis to make it clear what you want.
The "Left-to-right operator precedence" option on the General Settings pane allows you to control this behaviour.

\bf{

Operator  Description              Example

  !    NOT                        if (!a) 
  *    Multiply                   a = b * c;
  /    Divide                     a = b / c;
  %    Remainder                  a = b % c;
  +    Add                        a = b + c;
  -    Subtract                   a = b - c;
  <<   Bitwise Left Shift         a = b << c;
       (advanced users only) 
  >>   Bitwise Right Shift        a = b >> c;
       (advanced users only) 
  &    Bitwise AND                a = b & c;
       (advanced users only) 
  |    Bitwise OR                 a = b | c;
       (advanced users only) 
  ^    Bitwise XOR                a = b ^ c;
       (advanced users only) 
  ==   Is equal to                if (a == b)
  !=   Is not equal to            if (a != b)
  >    Is greater than            if (a > b)
  <    Is less than               if (a < b)
  >=   Is greater than or equal   if (a >= b)
  <=   Is less than or equal      if (a <= b)
  &&   Logical AND                if (a && b)
  ||   Logical OR                 if (a || b)

This order of precedence allows expressions such as the following to evaluate as expected:

if (!a & b < 4)

which will execute the 'if' block if a is 0 and b is less than 4.

However, it is always good practice to use parenthesis to group expressions. It's much more readable to script the above expression like this:

if ((!a) & (b < 4))