packages are separate naming context fora group of classes 
and interfaces.It provides a high layer of access 
protection and name space management.Theya re also used to 
organise related classes and interfaces into a single API 
unit and to control the accessibility of these classes and 
interfaces.
java.lang,java.lang.Throwable.javax.servlet.java.rmi

collection of classes and interfaces which can be reused in 
other applications are called packages...
java.lang,java.net,java.awt,java.util,java.io...
user defined packages can also be created
package <name>
//class or interface definitions....
class A
{
....
}
class B
{
...
}

Collection of class and interfaces stored under specific 
name is called as packages.

eg: ArrayList,Vector,HashTable are class available in the 
packahe java.util.

some of the packages are,
  java.lang, java.io, 
java.util,javax.servlet.http.HttpServlet.. etc.

package is just a namespace under which the
classes,interfaces are kept and they on the filesystem where
the application is running are just a folder and maintains
the files anf folders and help us to implement the
inheritance stuff and you can have 2 files with the same
name in two different packages which should clash, you can
call the class with full name packagage1.classname

Note Web-inf folders,Lib folders are not packages.
java.lang, java.util,java.io;