TestPlan contains a specialized testing language that we simply called "TPL". Files written in this language typically have the ".test" extension (which would explain why some docs may refer to the language as dottest still).

Note that TPL is only one of the features of the entire TestPlan framework. We encourage people to use it, though it is by no means a requirement -- extremely large web applications will likely require some Java units, though we're always working towards that not being necessary.

To understand why TPL exists, you may wish to read The evolution of .test as a language→.

For an introduction to the language you can look at An example using Google→. It is a very basic script, but gives the basic idea.