what is the difference between administrator session and developer session in salesforce. And also While I am searching for tutorials, I saw that 'Apex for Admins' etc..
A Salesforce administrator is someone who manages and administers a production Salesforce organization. Admins usually carry out minor declarative changes and have responsibility for managing releases into production.
A developer would typically build functionality including Apex/Visualforce in a sandbox before handing it over to the administrator to schedule deployment.