A computer cover is a thin, hard plastic shell that protects the computer from scratches and other damage. It covers the entire surface of the laptop and has cut-outs for all the ports, buttons, and screens. A laptop bag is designed to carry your laptop and other items you may need for work or school.
The best choice would be to use both. The cover will protect your laptop from scratches, while the bag will protect it from spills and other hazards.