As I understand it, there is no difference. The first immunization was for smallpox. Smallpox was deadly, but a similar virus, cowpox, was very mild in people. A doctor noticed that people who got cowpox often didn't get smallpox, even during an epidemic. So he inoculated (injected) people with cowpox to prevent smallpox, and it worked. As the Latin word for cow is vacca, this first immunization was called vaccination. Even though cowpox was not used again, the term stuck.
Immunization, in general, is done with dead or weakened (attenuated) versions of the virus. These inoculations are less dangerous than the disease, and activate the immune system to prevent the disease.
So immunization is the more accurate term, and the term vaccination is also still used for, as far as I know, the same procedures.