Black box testing is the Software testing method which is used to test the software without knowing the internal structure of code or program. Most likely this testing method is what most of tester actual perform and used the majority in the practical life.
Basically software under test is called as "Black-Box", we are treating this as black box & without checking internal structure of software we test the software. All testing is done as customer's point of view and tester is only aware of what is software is supposed to do but how these requests are processing by software is not aware. While testing tester is knowing about the input and expected outputs of the software and they do not aware of how the software or application actually processing the input requests & giving the outputs. Tester only passes valid as well as invalid inputs & determines the correct expected outputs. All the test cases to test using such method are calculated based on requirements & specifications document.