1)unit testing
2)integration testing
3)system testing
4)user acceptace testing

1)unit testing:
 individual software components of appication are tested in 
isolation from  other part of the program.
  

 
bigban testing: individual software components of an 
aplivation are combined at once into system .every module i 
s first init tested.then the entire sysetm is tested for 
communication interfaces between them.
 

bottom-up testing: 
  in bottom-up integration testing begins from bottom of 
the module hierarchy and work up to the top to simulate 
higher level modules .every module is first unit tested 
then modules are added in acsending hierarchial order.lower 
level modules are tested first then the next set of higher 
level modules are tested with previuosly tested lower level 
modules.


top-down testing: 
    begins testing from top of the module hierarchy and 
work down to the bottom to simulate lower interfacing 
modules.every module is first unit tested then the modules 
are added in descending hierarchial order.higher level 
modules are tested first then the next set of lower level 
modules are tested with previously tested higher level 
modules.

Unit testing: typically done by developers will do this, 
code can be tested.
Integration testing: This is can be done by after 
completion of unit testing. All dependent modules can be 
tested in this level; in here one module output is input to 
the next module,
Ex: login page to home page, home page to account transfer 
page.
System testing:  here all components of the system are 
combined together and test it. Here END-END flow of the 
system will be tested.
Acceptance testing: the customer does this; client will be 
using the system against the business requirements

Hi Venkat,
               Thanks for answering in the simple 
terminology. I think after unit testing, tester should 
begin with the smoke test and should make sure that all the 
links are present with out any breaks.

Unit testing:
     Unit is considered as the smallest module in an 
application.A unit contains codes of few lines,which 
performs a particular task.
     Testing the unit,testing the logic of the code is said 
to be unit testing.
     For performing this test we need Low level design 
document(LLD).And we perform white box testing.And the 
testers are called as Unit testers.

Hi Purni,
      
       Can u pls tel me what is a smoke test.For what 
purpose it is used?

smoke text is done once build is released to the testing 
team, to make sure that all the links are properly working 
without fail to move further level of testing