Tony

Suckers are upward growing branches that sprout at the base of the trunk. Another simular growth are water sprouts which are upward growing branches that sprout off of the primary branches. You'll be able to tell the difference between these and the regular growth becuase of way they grow. Their growth will be fast and coarse with long internodes and large leaves and they will extent upwards toward the sun. Most species of trees want to grow tall fast so the tree naturally will put more energy into these upward growing branches.

Good or bad?

You wouldn't want to use any of this growth for your final branches due to problems mentioned above. They can however be very handy in the training of trees. For instance, those suckers that sprout from the base of the trunk can be used for sacrifice branches to add thickness and taper to the trunk. You have to be careful here though that the suckers don't overtake the apex of the tree. If that happens you could lose the main trunk. Like I said, most trees just want to grow tall. They don't necessarily care how pretty they look getting there or if they have 1 or 6 trunks.

Good or bad? Neither really. You as the grower must learn to use the tree's energy to your best advantage.

Tony