Card stock

Card stock can be used for multiple purposes and is available in white or any other color.
Colored paper can be either solid or not. If it's not solid, it means that the color is printed on and that the "inside" of the paper is still white. This white core might show through if you tear or lightly sand the paper.
Kraft paper is a brown type of paper that's also available in different weights.

Card base

Card base is the name given to the piece of paper that makes up the actual card. This should be sturdy enough to allow the card to stand up nicely.
Keep in mind top folding cards (where the fold of the card is at the top rather than on the side) will need extra sturdy paper to keep the card standing up without the "legs" sliding apart.
If you're not going to use a specific coloring medium on your card base, the card base can be created from cheaper paper. As long as it's heavyweight.

Card front or card panel

A card front is the piece of paper/card stock you will stick onto your card base. It will be your main piece of art.
You can make the card front the same size as the final card, or you can make it a bit smaller (by subtracting the same number from both the width and the height of the card size) to give your card front a bit of a frame.

Colored images

Images can be stamped onto white paper and then colored in with one of many mediums such as alcohol markers, watercolor (paints, markers, pencils).
Depending on the medium you're using, you'll want to choose the right type of paper. This paper might be a bit more expensive since it's made for a specific purpose (allowing alcohol markers to blend, the ability to take more water …).