Planning is where you plan all your testing activities, meaning you setup your requirements and testcases.
Projects Everything belongs to a project. Projects have multiple users attached to them. Projects have multiple sites attached to them. Sites are the various URLs that the project uses eg. developement/production URLs. Add/edit the sites by editing the project.
Requirements work as containers for testcases as well as other sub-requirements. You can add requirements from the Planning menu.
You can manage requirements structure by clicking 'Manage requirements' which will enable you to drag 'n drop them on to each other, making them sub-requirements of each other.
Requirements contain the OS/browser combinations that contained testcases will be run in.
Testcases should be the humanly readable test of some functionality. A testcase can have a single testscript attached to it. Attach the testscript by using the upload testscript functionality. Testcases can have steps attached to them. Steps are a step by step description of the test, eg. open google.com, google.com opens. Add/edit steps when editing the testcase. Testcases must be linked to a requirement. A single testcase can be linked to multiple requirements. Use the 'Manage testcases' link to drag 'n drop testcases to the requirements they should be linked to.