Cricut uses the term "cartridge" to refer to image sets in general. Cartridges, or image sets, are groups of images that are related by a theme. Many cartridges are available in both physical and digital formats. Some physical cartridges have been retired, but the digital versions may still be available for purchase on Cricut.com. There are also some image sets that have been released in digital form only.
Many image sets have been released in physical form as a plastic cartridge that you can insert into a Cricut machine to access the image content. Physical cartridges can be linked to your Cricut account through Cricut Design Space. When a physical cartridge has been linked to your account, the digital versions of those images are activated on your account and the physical cartridge is no longer needed in order to use those images with Cricut Design Space.
Digital cartridges are image sets that are sold as a digital item only; nothing is shipped to you, but the image content activates on your account instantaneously once the purchase is complete, and then can be accessed when signed in to your account in Cricut Design Space.
Digital content, including cartridges, single images, and Cricut Access plans, will activate on whatever email address is used to complete the purchase.