Ok, lets start with Difference training:
There are seven major differences regarding class ratings:
1. Tail Dragger
2. Turbo Charged
3. EFIS cockpit
4. Single-Lever throttle
5. Constant speed
6. Retractable gear
7. Pressurization (pressurized cabin)
E.g. If you have a class rating on a Piper Arrow (Constant speed & Retractable gear), and you want to fly a Cirrus (Constant speed & EFIS cockpit), you need a difference training because there is a specific difference to your plane and the one you want to fly.
However, if you have a rating on a plane that has one or more of the above, and you want to fly a plane that is one short, you don't need difference training.
Now; Familiarization Training:
If you have a rating on a Piper Cadet (which has none of the 1-7 differences above), and you want to fly a Cessna 152 (which I believe also has none of the above), you need a familiarization course. Why? Speeds and/or SOP's will differ from plane to plane. High-wing/low-wing, wing-profile etc.
So if there is no difference between the plane on which you have the rating and the one you want to fly (hardwarewise, see 1-7 differences), you would only need a familiarization course.
Hope this helps
