What is the difference between shin splits and a stress fracture?

Muscle vs. bone. Shin splints are a condition due to a strain or inflammation of teh anterior tibialis muscle or tendon. Stress fracture is a small non-displaced fracture (break) of a bone due to repetitive trauma.
Same process. The both span the stress injury continum... Stress injury to a bone starts with painless inflammation (microfractures). When pain starts we call this shin splints. If one does not give this process some rest, it will culminate in a broken bone or stress fracture.