CATS means cross application timesheet which enables entries into time tables, the below link will help you in more detail.
CATS classic is the original user interface of the Cross-Application Time Sheet in the SAP system. It is particularly suited to users who also use the SAP system to perform other tasks. CATS classic is the most suitable user interface for entering working times for multiple personnel numbers, for example, by secretaries.
CATS Regular (ITS):
CATS regular (ITS) offers a similar data entry screen and comparable functions to CATS classic. However, this user interface has been optimized for use as an Employee Self-Service (ESS) application in a Web browser. In contrast to CATS classic, CATS regular (ITS) users only need a Web browser and not a SAP GUI for Windows. This makes CATS regular (ITS) the most suitable user interface for recording working times in the intranet or Internet.
CATS notebook can be used by sales people who are on site and do not have Internet connection. They enter their time sheet info offline into CATS notebook & when they are online, timesheet data is transferred to the server.
CATS for service providers enables you to specify basic data that is often valid for more than one day. You can then record task components (such as miles/kms driven, overtime, and so on)