Difference between Manual Testing and Automation Testing

• If you have to run a set of tests repeatedly, automation is a huge win for you.
• It gives you the ability to run automation against code that frequently changes to catch regressions in a timely manner.
• It gives you the ability to run automation in mainstream scenarios to catch regressions in a timely manner.
• Aids in testing a large test matrix (different languages on different OS platforms).  Automated tests can be run at the same time on different machines, whereas the manual tests would have to be run sequentially.

Cons of Automation

• It costs more to automate.  Writing the test cases and writing or configuring the automate framework you’re using costs more initially than running the test manually.
• Can’t automate visual references, for example, if you can’t tell the font color via code or the automation tool, it is a manual test.

Pros of Manual

• If the test case only runs twice a coding milestone, it most likely should be a manual test.  Less cost than automating it.
• It allows the tester to perform more ad-hoc (random testing).  In my experiences, more bugs are found via ad-hoc than via automation.  And, the more time a tester spends playing with the feature, the greater the odds of finding real user bugs.

Cons of Manual

• Running tests manually can be very time consuming
• Each time there is a new build, the tester must rerun all required tests - which after a while would become very mundane and tiresome.