The various types and levels of testing can be be define defined as Unit testing, Integration testing, Systems testing, Requirements testing, Beta testing, Stress testing, Regression testing, Reliability testing, Performance testing and Usability testing. These test types are are explained below:

Unit Testing
This is testing of compiled program code at the lowest level and is source code that does not include any called subroutines or functions. Fundamentally, every source statement is executed at least once during testing. The tests are performed to find unit execution problems, which represent the second most frequent type of bug.

Integration Testing
Here the testing of interfaces between otherwise correct components and modules is covered to ensure compatibility. Integration bugs are just behind Unit bugs in frequency although for Objected Oriented software, they may be more frequent.

System Testing
Testing an entire software system end to end to discover any common system bugs such as loss of resource, synchronization and timing problems, multi-user sharing problems etc. After fixing the low level bugs any system bugs can be addressed.

Requirements Testing
These will cover end to end tests from the User perspective to verify operability of each feature. It may not be good to presume that ‘user’ just means ‘end-user’. In typical software, only 10% - 15% of the code directly concerns things immediately seen by the end user. The remaining 85% concerns infrastructure items such as resource management, protocols, databases and files that the user does not know about or want to know about.

Beta Testing
Beta testing is usually done by representative users, typically in the final stages of testing prior to official release. A small representative sample, normally under 1%, of an installed user base can be an effective way to discover any latent configuration sensitivity and performance bugs not previously found.

Stress Testing
It involves subjecting a software system to an unreasonable load while denying it the resources needed to process the load. This can be one of the most economical ways to find system bugs. Proper stress testing is useful in finding synchronization and timing bugs, interlock problems, priority problems, resource loss bugs and general abuse of application program interfaces.

Regression Testing
This is more specifically Equivalency testing; i.e. re-running a suite of tests to assure that the current version behaves identically to the previous version except in areas where changes have been applied.

Reliability Testing
This will determine the expected failure rate of software under a statistically specified user load or operational profile. Reliability testing can give an effective method to determine when enough testing has been completed to warrant migration to production.

Performance Testing
This is the testing to determine the expected processing delay as a function of the applied load (and to determine the resource utilization under load). This testing may involve determination of the maximum simultaneous users or transactions that a system can sustain. Simple tests may be feasible. For example, inspection of memory being used or simple batch process job timings.

Usability Testing
This involves testing the human / machine interface including the screen menu layouts, help features, instruction manuals and icon style and placement in order to confirm that such things are well thought out and the system can be learnt and used with a minimum of effort. This type of testing should commence during early development stages otherwise in may be too late to make any substantive changes.