System settings contain the basic and advanced settings of your website.  The menu contains the following items:

Settings:  domain, culture, display name, SMTP and custom fields

Site users: users and their role in this site.

Custom errors: custom error pages of your site.

Url redirects: redirects to map old URL to new URL

Robots.txt: Rules used by search engine to crawl your websites

System diagnosis: assessments of your system's health