I'll try to answer the questions I can and leave the others to the experts. There are 2 basic types of die cutting machines - manual and electronic. Manual machines such as the BigShot need a die which cuts your paper to a specific size and shape. They can also be used to emboss (create a raised impression on one side of your cardstock). Electronic machines also cut your paper into various shapes. The difference is that you decide the size of the final product. Most machines, like the Cricut, use a cartridge which contains many images as opposed to a die which only cuts one image. This means that cartridges cost more than dies, but it also means that you would need many dies to equal what's available to you on a cartridge.
SCAL is compatible with the Cricut, however it does void your warranty if installed and used on the machine during the first year of ownership (if the Cricut is new). It was not created by the makers of the Cricut and is a third party software.
Embossing folders are in no way similar to carts (shortened form of cartridges). They are more like dies in that they only create one shape on your paper, but they don't cut out the shape. Think of them as a mini file folder with ridges on the inside. When you add paper inside and press down firmly the ridges get imprinted and now your paper has those same ridges too. Put a scrap piece of cardstock on a mousepad and draw a line with pen and flip it over. The raised line is the same effect you would get from an embossing folder.