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What do the built-in @Override, @Deprecated, and @SuppressWarnings annotations do? How do you apply multiple different annotations to the same element, and does their order matter? How do you create a custom annotation in Java, and what role do @Retention and @Target play? What is the difference between compile-time annotation processing (via javax.annotation.processing.Processor) and runtime reflection-based annotation reading? What are Java's meta-annotations (@Retention, @Target, @Documented, @Inherited), and what does each control? What is the purpose of the @FunctionalInterface annotation, and is it required for an interface to actually be treated as a functional interface? How do annotations with elements (parameters) work, including default values and the special `value` element? What is the difference between the @Nullable/@NonNull annotations (from libraries like JetBrains or JSR-305) and Java's built-in Optional type for expressing nullability? How would you read custom annotations at runtime using reflection to implement annotation-driven behavior, such as a simple dependency injection or validation framework? How do repeatable annotations work in Java (using @Repeatable), and what problem do they solve?
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What are annotations in Java, and what purpose do they serve?

Beginner

Answer

Annotations are a form of metadata that attach information to Java code elements (classes, methods, fields, parameters) without directly affecting program logic -- they're read by the compiler, tools, or the runtime via reflection to influence behavior, such as @Override triggering a compile-time check, or @Entity marking a class for an ORM framework to map to a database table.
@Override
public String toString() {
    return "MyObject";
}

@Deprecated
public void oldMethod() { }

@SuppressWarnings("unchecked")
public void legacyCode() { }
Real-world example A Spring Boot application uses @RestController and @GetMapping annotations to declaratively wire up HTTP routes to Java methods, letting the framework read this metadata at startup to build the routing table without any manual registration code.

Common follow-ups: What's the difference between a marker annotation (no elements) and one with elements?;How does the compiler use @Override to catch a common class of bugs?

Reflection API;Java Fundamentals: Syntax Data Types & Operators
All 15 Annotations questions