An Activity is an internal task that CatTools performs against one or many of your network devices that you have entered into the CatTools database.
It can be scheduled to run at specific times using the Timer or it can be run immediately by using the 'Run Now' button.
The Activity will launch clients to go out and query your devices and obtain the information needed to complete the task.
It will collect all of the information returned and then act upon it either storing it for future use or analysing it there and then.
It will then report to you via email the results.
A basic example would be to Backup the Running config of all of your network devices.
To do this you would :