Constructors in Java help

What is a constructor? How do you differentiate between a constructor and a default constructor? Please explain in detail, thank you.

The best detail is found in the Java tutorial with examples. The index is on the left side of the page.

Home page:
http://docs.oracle.com/javase/tutorial/index.html

Classes:
http://docs.oracle.com/javase/tutorial/java/javaOO/index.html

Constructors:
http://docs.oracle.com/javase/tutorial/java/javaOO/constructors.html