What is JDK (Java Development Kit)
JDK is a software development program provided by sun Microsystems. Java Development Kit or JDK comes in various version and can be downloaded free from the sun Microsystems. JVM compiler, debugger and other tools are used with JDK for developing java based application & java applets. So make sure that your JVM compiler & JDK versions are same.
JDK also known as Java 2 Platform, That comes in three editions J2ME, J2SE & J2EE. If you are beginner or learning Java then start by downloading J2SE.
JDK Java Development Kit
JVM Java virtual machine
You can download JDK from http://www.oracle.com/technetwork/java/index.html
Latest version of JDK
The latest version of JDK is 7. You should now download JDK 7. These are many new features in JDK 7. If you are using the Android Studio for development of Android applications then you should download the JDK 7 and install on your computer. After downloading the JDK 7 you should install on you computer and then configure the path and classpath variables.
Here are few tutorials about JDK 7:
JDK 5.0 Update 6
The full internal version number for this update release is 1.5.0_06-b05 (where "b" means "build"). The external version number is 5.0u6. Java Version 1.5.0_06 introduces various security enhancements in Java Plug-in and Java Web Start to better protect users and enterprises. For more information please visit: http://java.sun.com/j2se/1.5.0/ReleaseNotes.html