A test automation framework is a set of assumptions, concepts and tools that provide support for automated software testing

Test automation

Test automation is the use of software to control the execution of tests, the comparison of actual outcomes to predicted outcomes, the setting up of test preconditions, and other test control and test reporting functions...

. The main advantage of such a framework is the low cost for maintenance. If there is change to any test case then only the test case file needs to be updated and the Driver Script and Startup script will remain the same. Ideally, there is no need to update the scripts in case of changes to the application.

Choosing the right framework/scripting technique helps in maintaining lower costs. The costs associated with test scripting are due to development and maintenance efforts. The approach of scripting used during test automation has effect on costs.

Various framework/scripting techniques are generally used:

1. Linear (procedural code, possibly generated by tools like those that use record and playback)
2. Structured (uses control structures - typically ‘if-else’, ‘switch’, ‘for’, ‘while’ conditions/ statements)
3. Data-driven
   Data-driven testing
   Data-driven testing is a term used in the testing of computer software to describe testing done using a table of conditions directly as test inputs and verifiable outputs as well as the process where test environment settings and control are not hard-coded...
   
    (data is persisted outside of tests in a database, spreadsheet, or other mechanism)
4. Keyword-driven
   Keyword-driven testing
   Keyword-driven testing, also known as table-driven testing or action-word testing, is a software testing methodology for automated testing that separates the test creation process into two distinct stages: a Planning Stage, and an Implementation Stage.-Overview:Although keyword testing can be used...
   
   
5. Hybrid (two or more of the patterns above are used)

The Testing framework is responsible for:

1. defining the format in which to express expectations
2. creating a mechanism to hook into or drive the application under test
3. executing the tests
4. reporting results

Another view
Automation Framework is not a tool to perform some specific task, but is an infrastructure that provides a complete solution where different tools work together in an unified manner hence providing a common platform to the automation engineer using them.

Ref: http://code.google.com/p/vauto/