You can design with all of the cartridges, but you can only cut with the ones you have. Cricut Design Studio has every available cartridge loaded into the program. You can create all kinds of cutting files with any cartridge they offer. While that is fun for experimenting, just remember that when you go to cut the Cricut will ask you to load the specific cartridge used.
You can use more than one cartridge in a cutting. The options are endless. Bring letters and images from any cartridge into the cutting you are designing. When you are ready to cut, the Cricut will ask you to load each cartridge you used.
Connecting letters together to form one cutting is reffered to as welding. In order to weld letters together it is important to remember to check the welding box for each individual letter or image. Another interesting tidbit about welding is that it removes any scorelines on the image.