Osteoporosis is characterised by a loss of bone density. That is to say, unlike other conditions, such as osteomalacia, where specific parts of the bone matrix are lost, osteoporotic bone is of normal composition. Rather, there is less of it, making it weaker and more prone to fractures after only minor traumas (such as falling over).
In addition to a loss of bone there is a particular loss of horizontal trabecular parts of bone (see 'What is the structure of a typical bone?'), found at important joints, contributing further to the weakness of the bone.