If you're looking at boards, then "cradled boards" or "cradled panels" have a frame behind them (basically the same as the back of the canvas), which makes it easier to apply fittings later, if you plan on putting them on a wall or anything. A flat board (i.e 6mm plywood) on its own would tend to need framing, as it's not deep enough to hold most screws.

For gesso, simplifying things a bit, it's pretty much a cross between a primer/undercoat and a filler. If you buy a pre-made canvas, it's normally been pre-gessoed/pre-primed. A cradled board is likely to be bare wood. If you want to avoid buying extra stuff, you can get away with a couple of thin coats of white acrylic paint as an undercoat and not worry about it too much :)

For pre-painting taping, I only realised that after years - I saw someone else do it and thought "oh yeah, that makes a lot of sense!".