In Java, annotations are a form of metadata that provide additional information about program elements such as classes, methods, fields, and parameters. Annotations are defined using the '@' symbol followed by an annotation name and can be added to a program element by including them in the source code.

Annotations are typically used to provide information to the compiler, tools, or runtime environments about how to handle or process the annotated elements. For example, the '@Override' annotation can be used to indicate that a method is intended to override a method in a superclass or interface. Other common annotations in Java include '@Deprecated', which marks a program element as obsolete or outdated, and '@SuppressWarnings', which suppresses compiler warnings for a particular program element.

Annotations can also be used to define custom metadata and are often used in frameworks and libraries to provide additional information or configuration options. To define a custom annotation, you would use the '@interface' keyword followed by the name of the annotation and any properties or values that it requires.

Custom annotations, also known as user-defined annotations, are annotations that are defined by a Java developer for their own use. These annotations can be used to provide additional metadata or to mark certain elements of a program for special handling or processing by the developer's own code or third-party libraries and frameworks.

To define a custom annotation in Java, the developer uses the '@interface' keyword followed by the name of the annotation and any properties or values that it requires. For example, the following code defines a custom annotation named 'MyAnnotation' with two properties:

import java.lang.annotation.*;

public @interface MyAnnotation {
    String author();
    String version() default "1.0";

In this example, the '@Retention' and '@Target' annotations are used to specify the retention policy and target element type for the custom annotation, respectively. The 'author' and 'version' properties are then defined as string values, with the 'version' property given a default value of "1.0".

Once a custom annotation is defined, it can be used to annotate program elements in the same way as built-in annotations, using the '@' symbol followed by the name of the annotation. For example, the following code annotates a method with the 'MyAnnotation' custom annotation:

@MyAnnotation(author = "John Doe", version = "2.0")
public void myMethod() {
    // method body

Custom annotations can be a powerful tool for Java developers, allowing them to define their own metadata and communicate important information about their code to other developers and tools.