Manual Testing involves finding errors or bugs in a software application without using any automation tools in particular.
New Software applications must be manually tested before an automated test plan can be devised and executive. Since the tester has to go through all the test cases manually in case of manual testing, so it requires more effort and time.
An exhaustive test plan is created in the first place which has test cases to be executed to determine the quality of product being tested. Manual testing is usually the first step towards the process of QA, as it gives out a clear picture of nuances of testing a particular product.
Lot of times, a basic exploratory testing is enough for small teams and projects wherein a tester explores the user interface of the application and uses all the features of the product intuitively, without following any rigrous procedure or test plan. The tester in this case builds upon the test cases logically on the fly and documents the errors on parallely.
In a larger setup a more rigrous and formal approach towards manual testing is applied. There is a test plan with test case which cover most of product's functionality.