If you are wondering what the differences are between Project Online and Project Server 2013, here's a useful link. Project Online and Project Server 2013 on-premises are based on the same set of features and functionality, but there are differences between the two. The following table provides an overview of the differences between Project Online and a Project Server 2013 on-premises environment.

http://technet.microsoft.com/en-us/library/dn268595.aspx