Clicking the main navigation link for Contacts opens a list of individuals in a new tab in the main interface. These are the representatives of the organizations you deal with, or wish to deal with - clients & prospects can have contacts associated with them, leads are something of a special case, they combine both organization and individual. Leads can only be used by Synergist user accounts with a sales & marketing license. You can find out more about creating and managing leads in the sales & marketing section of this help center.
The list of contacts displays basic information and has various controls common to lists in Synergist, read more about common user interface controls in this article. Click on an item in the list to open the detail input form for that contact in a new tab in the main interface: