multitasking infers the mechanism to run many processes 
simultaneously with user interaction.
                             in contrast,multithreading is 
a mechanism of running various threads under single process 
within its own space.

multitasking:heavyweight process and runs in a different 
address space so context switch or intercommunication 
between processes is much expensive.

Where as multithreading: light weight process and can run 
in a same address space so context switch or 
intercommunication between processes is less expensive.

multitasking ::: is a method by which multiple tasks, also
known as processes, share common processing resources such
as a CPU. With a multitasking OS, you can simultaneously run
multiple applications. Multitasking refers to the ability of
the OS to quickly switch between each computing task to give
the impression the different applications are executing
multiple actions simultaneously.

Multithreading ::: extends the idea of multitasking into
applications, so you can subdivide specific operations
within a single application into individual threads. Each of
the threads can run in parallel. The OS divides processing
time not only among different applications, but also among
each thread within an application.

MULTITASKING:
              multitasking ,in an operating system is 
allowing user to perform more than one computer task at a 
time.
MULTITHREADING:
              the ability of an operating system to execute 
different parts of the program called 
threads,simultaneously.the programmer must carefully design 
the program in such a way that all the threads can run at 
the sametime without interfering with each other.

A multi threaded program contains 2 or more threads 

running concurently.
In a multitasking environment 2 or more processes will run 
concurently.
            Multithreading requires less overhead than 
multitasking.
 
             Interprocess communication is very expensive 
and context switching from one process to another procees 
is costly since they are runningin different address spases.

In case of multithreading threads are lightweightprocess and
can share same addressspace and interthread communication 
is less.

expensive
than interprocees

multitasking :multitasking is a method by which multiple 
tasks, also known as processes, share common processing 
resources such as a CPU. With a multitasking OS, you can 
simultaneously run multiple applications. 

Multithreading : extends the idea of multitasking into
applications, so you can subdivide specific operations
within a single application into individual threads. Each of
the threads can run in parallel. The OS divides processing
time not only among different applications, but also among
each thread within an application

Multitasking is of two types
1> process based.
2>thread based.

MULTITASKING:
       multitasking ,in an operating system is 
allowing user to perform more than one computer task at a 
time.
Multithreading: 
Multithreading is a mechanism of running various threads 
under single process 
within its own space.