If you look in the database you will see the tables named
CMS_<tablename>
or
COM_<tablename>
The standard practice of the Kentico API is the <tablename> is the Info object. So if we take CMS_Site for example and create a new site object in code you'd write
SiteInfo si = new SiteInfo();
si.SiteName = "Your site name";
If you want to get or update data you use the Provider. Each <tablename>Info object has a provider.
DataSet sites = CMS.SiteProvider.SiteInfoProvider.GetAllSites();
Here is a full code example
// create a new site object
CMS.SiteProvider.SiteInfo si = new CMS.SiteProvider.SiteInfo();
si.SiteName = "MySite";
si.DisplayName = "My Site";
si.DomainName = "localhost";
// insert the site object
CMS.SiteProvider.SiteInfoProvider.SetSiteInfo(si);
//get a list of the sites
DataSet sites = CMS.SiteProvider.SiteInfoProvider.GetAllSites();
So to follow suit, look at the user table, CMS_User. The User object is UserInfo. To get user info you use the static method UserInfoProvider.GetUserInfo(56);
Good luck!
Brenden