In a post about renovating mature pear trees there is a diagram (below) showing tree nomenclature as it pertains to pruning. In it, the letter "A" indicates suckers, and "E" marks shaded interior branches. What's the difference?
Another diagram I found references water sprouts - what are they and how are the different from the suckers or vertical branches?
IMHO they are pretty much the same thing (with various names, as happens in English), except that E has been let go long enough to make branches of its own (should have pruned more water sprouts last year.)
Regardless of what they are called, they should be removed.
Stark Brothers defines sucker as root-derived, water sprout as coming from the tree above-ground (so only the bottom two A's are suckers by their definition) Virginia Tech agrees (and that's their drawing below) so I'd suspect that this is generally accepted terminology in the industry, which the drawing you linked to does not happen to conform to.
Suckers are root based growth and especially detrimental on grafted trees.
They rob the desirable part of the tree above the graft of resources and if allowed will supercede and kill the graft. Once this happens, the tree reverts back to its hardier but less desirable rootstock species.
