Cliffhanger

'''A type of plot device that occurs either at the end of a chapter/division or at the end of a story that eludes to the next story and/or doesn't answer all the questions of the story. '''

A good source of cliffhanger examples can be found at the end of most season finales of TV shows.

End-of-Chapter Cliffhangers
This type of cliffhanger happens at the end of chapters, encouraging the reader to read on. The question that this cliffhanger makes you ask should be answered in the next chapter or two. If it has multiple POVs, you can get away with answering it several chapters away.
 * The best way to do these is to threaten to kill a protagonist. What this means is that at the end of the chapter, put a protagonist in a life-or-death situation (maybe even a literal cliffhanger). Then, in the next chapter, or the one when you answer the cliffhanger, show how they lived or died.

End-of-Story Cliffhangers
This type of cliffhanger happens at the end of a story, which encourages the reader to pick up the next book. However, you must not have an end-of-story cliffhanger without a sequel to carry it on. This annoys the readers.
 * The best way to do these is to relate the cliffhanger to a/the antagonist. Maybe you could reveal that the villain is still alive.
 * Maybe you could introduce a new villain, or you could reveal that another thought-to-be-dead character is alive.