BlueJ is a development environment that allows you to develop Java programs quickly and easily. Its main features are that it is:
Simple BlueJ has a deliberately smaller and simpler interface than professional environments like NetBeans or Eclipse. This allows beginners to get started more quickly, and without being overwhelmed.
Designed for teaching BlueJ is deliberately designed with good pedagogy in mind. There is a popular textbook designed for teaching introductory university/college courses with BlueJ, and a site full of teaching resources.