difference between task and process model

Tasks are created in a node "user task" within a process model. So they are used within a process model.  AFAIK that's the biggest difference from the two.

how task generate in user task and what will occur in script task

Hi Shifa! A User Input Task allows an interface (a form) to be assigned to another user as part of a process. These are commonly used when needing to get earlier entered information reviewed by another user. Conversely, a Script Task is a system task that has no user interaction. A User Input Task requires a user to complete the node whereas a Script Task is completed by the system. 

Hi Shifa! A User Input Task allows an interface (a form) to be assigned to another user as part of a process. These are commonly used when needing to get earlier entered information reviewed by another user. Conversely, a Script Task is a system task that has no user interaction. A User Input Task requires a user to complete the node whereas a Script Task is completed by the system. 

Learn more about these types of Smart Services in our docs > docs.appian.com/.../Smart_Services.html