What's the different between pingpong and cycle?

If you use Ping Pong, the repeating pattern is A B C B A B C B A

The animation bounces back and forth.

If you use Cycle, the repeating pattern is A B C A B C A B C.

The animation repeats.

If you use Offset, the pattern continues like this: A B C D E F G and so on for ever. Use "Offset" on position with the layer moving from left to right pixels in 10 frames, and the layer will move from left to right forever. Use "PingPong" and the layer will move left 10 pixels, then right 10 pixels, then left again and again. Use "Cycle" and the layer will move left to right 10 pixels, then snap back 10 pixels and start again and again. 

That should clear things up.

What property are you applying it to? What values? You need to clarify. A screenshot of the graph editor with the post-expression graph enabled might also help.

Mylenium