Welds are how you fasten two blocks together. There are many reasons to do this, such as creating environmental details, or creating more complex blocks out of the ones available.
Hinges connect two blocks at a single pivot, making a free-swinging elbow-joint. This joint is ideal for swinging vines and other hanging objects.
Spring joints are similar to hinges, except they try their darndest to keep the joint at its default position. They're especially useful for attaching leaves to trees.