Hi Charlotte! I think you'll find that most canvasses are ready to accept paint; either Acrylic or Oil, so don't need priming with anything.
Deep backings are a personal choice, and these are often known as "Block Canvas", and can be hung either framed or unframed (Like Next prints). Normal thickness stretcher Canvasses are generally best framed, but there is no written law on this! I often complete an Acrylic painting and simply paint the edges black. It can then be hung as it is, or framed by the purchaser.
A Canvas Pad is just like a pad of Watercolour Paper, but the leaves, or pages, are meant for painting with Acrylic Paints. If you use these, the painted Canvas Paper would need to be mounted onto a board and, probably, under glass.
As far as the roughness or smoothness of the weave, this will, as you say, depend upon the painting; if the painting is detailed, then perhaps a smooth weave would be used, or a looser, less detailed painting may suit a rogher weave.
I hope this answers your questions, but come back to me with any other information and I'll always do my best to help.
Good luck and very best wishes,
PA & RSC Wigan area