In Java, annotations are a type of metadata that can be added to a class, method, or variable declaration, among other places, to provide additional information about that element. Annotations are defined using the @ symbol, followed by the name of the annotation and any optional parameters in parentheses. Here are some examples of built-in annotations in Java:

1. @Override: This annotation is used to indicate that a method in a subclass is intended to override a method in its superclass. If the annotated method does not override a method in the superclass, the compiler will generate an error.

2. @Deprecated: This annotation is used to indicate that a method or class is no longer recommended for use and is likely to be removed in a future release of the software.

3. @SuppressWarnings: This annotation is used to suppress compiler warnings for a specific piece of code, such as an unused variable or an unchecked cast.

4. @FunctionalInterface: This annotation is used to indicate that an interface is intended to be a functional interface, meaning that it has only one abstract method and can be used with lambda expressions.

Annotations can also be used to define custom metadata for your own code. You can create your own annotations using the @interface keyword followed by the name of the annotation and any optional parameters. For example, here is a custom annotation that could be used to mark a method as being thread-safe:

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.METHOD)
public @interface ThreadSafe {
    String value() default "This method is thread-safe";
}

In this example, the @Retention annotation specifies that the annotation should be retained at runtime, and the @Target annotation specifies that it can be applied to a method. The @interface keyword is used to define the custom annotation, and the value parameter is used to provide a default string value for the annotation. To use this custom annotation, you would apply it to a method like this:

@ThreadSafe("This method is thread-safe and has been tested extensively")
public void doSomething() {
    // method body
}

In this example, the @ThreadSafe annotation is applied to the doSomething() method with a custom message that overrides the default value. The annotation can then be used by other developers or tools to provide additional information about the method.