The if and if-else selection structures are both used to make decisions in programming. In an if statement, a condition is evaluated, and if the condition is true, the subsequent block of code is executed. If the condition is false, no code is executed unless an alternative is explicitly provided. The if-else structure, however, provides an alternative block of code to execute when the condition is false. In other words, while if executes only when a condition is true, if-else ensures that one of two blocks of code is executed depending on the truth value of the condition.