Surveys are the individual questionnaires that you send to participants, which are created as forms in the Study Structure tab. 

Survey packages act as digital folders for your surveys, and allow you to send them to an email address. Whether a package contains a single survey form or many, surveys are always sent via packages. You can learn how to create a survey package in the manual article - Create a new survey (package).

Survey package invitations are survey packages that are planned or instantly sent to a participant's email address. The data filled in by the participant will be contained in the relevant survey package invitation. You can learn how to create a survey invitation in the article - Send surveys

The survey package invitations are listed in the overview provided in the Surveys tab. The survey package you select when creating an invitation is what appears as survey 'name' for the participant. The individual surveys and forms are shown in the form navigator while answering a survey, if you have selected to display the form navigator when setting up the survey package.

Castor Connect is Castor’s mobile application, enabling participants to complete their study assessments (‘surveys’) from the comfort of their personal devices. As long as participants have been able to download the latest survey package, they will be able to continue to complete their assessments offline, then submit when connection is restored. The Castor Connect app can be enabled for both existing and new studies. Enabling the Castor Connect app for a study has no impact on existing survey features and functionality. Devices running Android 5.1.1 and upwards and iOS 11 and upwards are supported. We estimate that 95%+ of mobile devices in use today will be capable of using the application.