The spring source code examples use maven tomcat plugin and i am able to run these projects using the command mvn tomcat:run but what i dont see the application is not deployed in local tomcat server("C:\apache tomcat\webapps\")
The command tomcat:run starts an embedded Tomcat, it doesn't deploy an application to your local Tomcat (why does it matter anyway?).
And how it is different from cargo plugin?
Cargo is a container agnostic tool so you could configure it to use a remote container, a locally installed container, an embedded container (as long as implementations are provided for a given container) and still use an unified API (or set of commands).
In this case when I ran the command mvn cargo:redeploy, I see the application deployed in the location "C:\apache tomcat\webapps\"
Cargo doesn't support running Tomcat+6.x in embedded mode. So you are very likely using it with a locally installed container and your app gets physically deployed on it.
If i add cargo plugin to the POM of spring examples (like mvn-showcase,petclinic)..will it screw up the things?
It won't screw up anything, why would it.