Anatomy modeling requires in-depth knowledge of the anatomical properties of whatever creature you are working on. For humans, there is a plethora of knowledge available. See also Sculpting Anatomy. Another resource applicable to anatomy modeling is the Stretches section.
Deformation areas such as joints that bend or rotate should be modeled with care. They should consist of quads only in those areas (as much as possible) to ensure proper rigging and deformation when animated. This is not to say that there cannot be triangles, however with triangles on character meshes it can be more difficult to determine bone influences when rigging and create smoother deformations when animating. So use care and speak with your riggers and animators when approaching these types of meshes.