Paper that has a special coating on the surface to enhance print quality. This applies to both ink-jet and laser papers.
It comes in many different finishes including: Hi-gloss, regular gloss (think magazine), and silk or matte.

Difference between text and cover
Look at paperweight infographic
Text: copy paper. Light enough to fold, but heavier text weights can handle heavy 2-sided ink
coverage. Use this for letterhead, brochures, or layering into cards.
Cover: Otherwise known as Cardstock. This is a much heavier sheet. Should be tough to fold.
Depending on the weight, you need to see how much your printer can handle. Good for business
cards, postcards, greeting cards, or anything you want to stand out.
Envelope sizes
Look at the envelope size infographic
Paper weights
Look at the paperweight infographic