The test pyramid is a concept developed by Mike Cohn, described in his book Succeeding with Agile. Its essential point is that "you should have many more low-level unit tests than high level end-to-end tests running through a GUI".
Tests that run end-to-end through the UI are: brittle, expensive to write, and time consuming to run. So the pyramid argues that you should do much more automated testing through unit tests than you should through traditional GUI based testing.
The pyramid also argues for an intermediate layer of tests that act through a service layer of an application, what I refer to as SubcutaneousTests. These can provide many of the advantages of end-to-end tests but avoid many of the complexities of dealing with UI frameworks. In web applications this would correspond to testing through an API layer while the top UI part of the pyramid would correspond to tests using something like Selenium or Sahi..
The test pyramid comes up a lot in Agile testing circles and while its core message is sound, there is much more to say about building a well-balanced test portfolio. In particular a common problem is that teams conflate the concepts of end-to-end tests, UI tests, and customer facing tests. These are all orthogonal characteristics. For example a rich javascript UI should have most of its UI behavior tested with javascript unit tests using something like Jasmine. A complex set of business rules could have tests captured in a customer-facing form, but run just on the relevant module much as unit tests are.
In particular I always argue that high-level tests are there as a second line of test defense. If you get a failure in a high level test, not just do you have a bug in your functional code, you also have a missing unit test. Thus whenever you fix a failing end-to-end test, you should be adding unit tests too.
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