Control structures alter the path the Ruby interpreter takes. They include conditional expressions such as if, if/else and case, and the various loops.
A "boolean" expression is any expression that is intended to be evaluatated as true or false. Any expression may be used as a boolean expression, but generally only expressions with comparison operators are used as boolean expression.
While simple boolean expressions will get you pretty far, you'll soon find yourself wanting to act on more than one condition. For that you'll need boolean operators.
Conditional statements are what makes computer programs tick. Without conditional statements, all computer programs would follow the same path from beginning to end.