When you sign up for the trial version, you will start with Pro but can try each version.
Great for simple layouts with one or more full page image or mp4 video. Manage your displays via the cloud.
Standard adds the ability to customize the layout with multiple "zones" which can be text, slideshows, rectangles, videos and music.
Pro gives you the option to add widgets and multiple layouts to a playlist.
Enterprise is for larger organizations needing multiple user logins with different permissions and the ability to monitor device uptime.