Often times these two terms are used interchangeably, but they do mean different things, and that can be very important for your project!
Roving is used to describe fibre that has been carded and then pulled into a long strip. The fibres are going in every direction, the preparation is less dense and may have a more fuzzy appearance. Roving is considered a Woollen prep for spinners. Roving is popular in felting as the fibres going in all directions assist in helping the fibres felt together faster.
A Batt is fibres that have been carded, but left like a sheet instead of pulled into a strip.
Roving and Batts are popular for items that need more structure - needle felted sculptures, bags cat caves etc.
Combed Top is used to describe fibres that have been combed parallel, either by hand or machine. The fibres being parallel results in a smoother appearance, and can often feel more dense than roving. Combed top is considered a Worsted fibre prep for spinners. In felting, combed top can take a bit longer than roving to felt, but is popular for garments in Nuno Felting, especially the 19m Merino.
Please feel free to add info in the comments or ask questions and I'll add more info tot he post so it's as helpful as possible!
Check out Chaotic Fibres Roving, Batts and Combed Top