What is most-common SharePoint job roles ?

You do not need to know everything about how SharePoint works to be able to do this role, but you do need to know how Site, Lists and Libraries work and be able to manage the content for your users. This role is becoming more predominant as the Federated Support Model is being implemented into larger companies.

SharePoint provides central management, governance, and security controls. SharePoint platform manages Internet Information Services (IIS).

SharePoint Developer

For development of custom web parts and customization of OOTB web parts.

SharePoint Administrator

For adminstration of SharePoint web portals

SharePoint Consultant

For above both roles as well as for installing and building the whole architecture of SharePoint server.

SharePoint Developer, SharePoint Administrator, SharePoint Architect, and SharePoint Consultant.