As guyverix says, the official documentation answers most of these questions, but it is a bit opaque if you are new to Zenoss.
1. What is the difference between modeler plugin and Template.
Modeler plugins collect CONFIGURATION information - like interface details, filesystems, what monitored processes are running. They are run by the zenmodeler daemon, typically every 12 hours.
Templates collect PERFORMANCE information for those devices and components that have already been discovered by modelers. Typically, templates run every 5 minutes.
So, for example, a modeler will discover that you have filesystems of / and /home; a template will then collect the data showing bytes used, inodes used,..
2. How to configure the monitoring parameters for specific monitored servers.
This depends totally on what you want to monitor. Look at the monitoring templates supplied as standard and this will get you a long way.
3. what all monitoring available default?
Start from ADVANCED -> Monitoring templates and explore.
4. What scenario we will go for zenpacks?
Depends on what you need! Start on your requirements. See if the standard Core product satisfies them. If not, look for ZenPacks that provide the add-on functionality that you require. You can find the community ZenPacks at http://wiki.zenoss.org/Category:ZenPacks . There also appears to be a new AWS Zenpack announced today.
5. Is there any detailed doc which explains the indepth of the architecture of zenoss?
See the Admin Guide.
Cheers,
Jane