Alpha testing is done by developers after the development
phase is completed.
Beta testing is done in the absence of developers in user site.

Alpha testing is an internal acceptance testing. This is 
simulated or actual operational testing by users/customers 
or dedicated test team at the developer's site.

Beta testing comes after alpha testing. Versions of 
developed sotware called beta versions are released to 
limited audience outside of the company. Sometimes these 
versions are released to the public to increase feedback 
fields for the future users.

Alpha Testing:
It is carried out on the developer’s premises itself 
	in a controlled environment.
 beta Testing:
 It is carried out at one or more user’s premises 
  using their infrastructure in an uncontrolled manner.

Alpha Testing :- It is done after 80% of bugs are fixed.It 
is needed to check whether we can meet the deadlines within 
the time or not.
Beta Testing:-  It is done after 95% of code is finished. 
Just to check the application whether we can freeze the 
code or not.