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A control structure is a programming concept where a block of codes is executed selectively or repeatedly based on a specific condition. It is an essential feature in any programming language, as it allows the program to follow different execution paths and make decisions depending on defined conditions.

There are three basic types of control structures: Sequential, Selection, and Iteration. 1. Sequential structure: This is the simplest control structure where the statements are executed sequentially in the order they appear in the code. 2. Selection (also known as Decision) structure: This structure allows the program to make a decision based on a specific condition, so it can choose between multiple execution paths. The most common selection structures are 'if', 'if-else', and 'switch' statements. 3. Iteration (also known as Loop) structure: This structure allows the program to repeat a section of code several times, as long as a certain condition is met. Common iteration structures include 'for', 'while', and 'do-while' loops.

Let's take a simple example of a selection structure using an 'if-else' statement in a program that calculates the grade of a student based on their test score. ``` int testScore = 85; char grade; if (testScore >= 90) { grade = 'A'; } else if (testScore >= 80) { grade = 'B'; } else if (testScore >= 70) { grade = 'C'; } else if (testScore >= 60) { grade = 'D'; } else { grade = 'F'; } System.out.println("The student's grade is: " + grade); ``` In this example, the program checks the value of 'testScore' and assigns the appropriate grade based on the specified conditions. The 'if-else' statements are a selection control structure that allows the program to choose the correct execution path to determine the student's grade.