Would like to know the difference between icontrol and irules?

The difference really lies in where each is relevant and an understanding of the difference between the "management plane" where you configure the device itself, and the "data plane" where network traffic flows. The management plane is where you create VIPS, pools, profiles, and even iRules. For that you use tools like the management GUI, the TM shell (tmsh), and iControls. The data plane is where network requests are received and processed, and where iRules evaluate and manipulate this data in real time. The two APIs are essentially very much different and each has its own vastly different purpose.