Atlassian Crowd provides single sign-on between Atlassian Products i.e. a user will still need to provide their credentials at least once via the usual login screen when accessing one of the applications. Once authenticated, access to other applications connected via Crowd SSO is possible without re-entering the credentials. Once the underlying token expires the user will need to re-authenticate.
On the contrary, EasySSO performs single sign-on between the user's Windows workstation and an Atlassian Product using information already provided when the user logged in to their workstation. No credentials are being asked and once the underlying session expires, the user is re-authenticated transparently with no additional actions required. In essence, EasySSO provides "auto-login" functionality for Atlassian applications.
If the user navigates to another Atlassian Product with EasySSO enabled - they will be automatically logged in there. Though this process is completely independent of the first application, to the user it seems that they are always logged in to all Atlassian Products.
Atlassian Crowd and EasySSO do work together, but only if Atlassian Crowd is used as a user directory in the backend.
If (before you discovered the magic of EasySSO!) Atlassian Crowd was used for single sign-on then the Atlassian Crowd SSO configuration MAY interfere with EasySSO.