Here is my Official Definition of a Cliffhanger:
"A cliffhanger is a mini-climax without a resolution."
In a story, the climax is the most exciting part of the book, where every conflict and crisis the character faces has welled up into a tsunami that they must somehow fight back. In normal language, it's when everything goes wrong.
The resolution is when everything is just peachy after the climax and all the problems have been, well, resolved.
If you have ever read a book that has a climax but no resolution, then you have read a book that ends on a giant cliffhanger. Ember Falls by S.D. Smith is a good example.
I find that a good spot for a cliffhanger is the end of a chapter. If you are writing a series, then you could end all the books but the last on a cliffhanger.
Try this technique. Next time you write a story, try to include a cliffhanger.