A pre-filled cart is the same as an empty cart. It has just been filled at the source of manufacture with a standard (usually tobacco flavor) juice. You can refill a pre-filled cart just the same as you can an empty cart. The empty cart comes with filler material or it may not have the filler material. You might want to ask that before you purchase.
510 family ecigs are standards around here. Some also use the 306 and 808 models as well as others. All have refillable carts.
A cartomizer is a different animal. The cartomizer is sealed and has been filled at the factory. Most cartomizers are designed to be used once and thrown away. Most of these can be refilled using a syringe. You usually buy them in packs of 5 or so.
Almost everyone fills their carts and cartomizers at home. It's cheaper and you get to choose the juice you put into them. Some people have carts that they fill and label with specific juices so that they can change at will to another juice. They usually have atomizers devoted to a particular juice or family of juices so that they don't mix flavors. Others just change from one juice to another by topping off the cart and allow the atomizer to use up any residue of the previous juice. Then you begin to taste the new juice you added to the cart.
The fillers used in carts range from polyfill that you can buy at Walmart to a blue foam sold for fish tank filters. There are lots of sources for information on the ECF forum. Just search using keywords and read the threads.
Here is a link to the ECF library containing lots of information with pictorial articles of everything you need to know.
The ECF Library