An effect uses textures (and models if needed) as input and transforms to the output (screen or rendertarget):
a bloom effect, a tonemapping, a depth of field, a ship with normal mapping etc,
It depends on what you want to do with the texture, if what you intend is just drawing a 3d model for ex, one effect (basiceffect for a start) for all models is enough, changing the effect's parameter to use one or another texture for example.
So the 2nd question is irrelevant for me
You set the texture one time for the effect, draw all you need to with this texture, then change the texture to draw another thing...
I'm not sure to be clear enough but it's the idea.