While HTML provides structure and CSS provides style, JavaScript provides behavior -- it's the only one of the three that can react to user actions, change content after the page loads, and perform calculations. Variables, declared with let or const (modern alternatives to the older var), store data such as numbers, strings of text, booleans that are true or false, and arrays, which are lists of values; functions are reusable blocks of code, written as function greet(name) { return "Hello " + name; }, that run when called. Conditional logic using if/else statements lets a program make decisions -- for example, checking whether a form field was left empty before allowing a submission -- and is one of the first concepts that makes code feel genuinely interactive rather than static.