Multithreading, Multitasking and Asynchronous processes

A task is one complete program so multi-tasking means the execution of two or more programs at the same time for example google chrome  uses multi-tasking  when there are many  open tabs even there are in  the some window. 

On the other hand, there can be several threads in one program. For instance, a video clip can have separate threads for video and audio. Multi-threading depends on the ability of the processor.

for the the Asynchronous  

When performing an asynchronous operation, you execute it using other threads.  

So to resume a task uses a thread to perform operation  

and  asynchronous operation uses a thread to perform an execution 

   Thread definition from Wikipedia 

              a thread of execution is the smallest sequence of programmed instructions that can be managed independently by an operating system scheduler