OCSlim is a port of Slim for Objective C. It's purpose it to enable the writing of acceptance tests for Objective C projects. To understand what Slim is, you need to know what Fitnesse, FIT and Slim all are.
To first understand what Slim is, you first need to understand what Fitnesse is. Fitnesse is the HTML and wiki front end for both FIT and Slim. FIT and Slim are different test runners built into Fitnesse.
Fitnesse is what you will use to create and run your tests.
FIT was the original test runner built into Fitnesse. Slim has since been added as an alternative. On the fitnesse webpage, there are many examples of FIT tests, which look very similar but are not identical to Slim tests. This is something to watch out for!
As previously mentioned, Slim is an alternate test runner to FIT. Slim itself is much more easily portable to a wide variety of languages.
Fitnesse is a wiki in which you will create and run your Slim tests. Slim simply defines the tests that you can write. Objective C Slim is the port of Slim for Objective C. It hooks your tests created in the wiki to your Objective C code.