Intent

Specify the kinds of objects to create using a prototypical instance, and create new objects by copying this prototype.

Explanation

Real world example

Remember Dolly? The sheep that was cloned! Lets not get into the details but the key point here is that it is all about cloning.

In plain words

Create object based on an existing object through cloning.

Wikipedia says

The prototype pattern is a creational design pattern in software development. It is used when the type of objects to create is determined by a prototypical instance, which is cloned to produce new objects.

In short, it allows you to create a copy of an existing object and modify it to your needs, instead of going through the trouble of creating an object from scratch and setting it up.

Programmatic Example

In Java, it can be easily done by implementing Cloneable and overriding clone from Object

class Sheep implements Cloneable {
  private String name;
  public Sheep(String name) { this.name = name; }
  public void setName(String name) { this.name = name; }
  public String getName() { return name; }
  @Override
  public Sheep clone() throws CloneNotSupportedException {
    return new Sheep(name);
  }
}

Then it can be cloned like below

Sheep original = new Sheep("Jolly");
System.out.println(original.getName()); // Jolly

// Clone and modify what is required
Sheep cloned = original.clone();
cloned.setName("Dolly");
System.out.println(cloned.getName()); // Dolly

Applicability

Use the Prototype pattern when a system should be independent of how its products are created, composed and represented; and

when the classes to instantiate are specified at run-time, for example, by dynamic loading
to avoid building a class hierarchy of factories that parallels the class hierarchy of products
when instances of a class can have one of only a few different combinations of state. It may be more convenient to install a corresponding number of prototypes and clone them rather than instantiating the class manually, each time with the appropriate state
when object creation is expensive compared to cloning

Real world examples

java.lang.Object#clone()

Credits

Design Patterns: Elements of Reusable Object-Oriented Software