There are many types of divination (also known as fortune-telling) in the world; humans have been attempting to infer messages from the greater universe since ancient times. The legendary Greek Oracles of Delphi, who breathed in noxious gases and then related the hallucinatory visions they received, are just tip of the iceberg. The earliest known form of writing from what is now China, which dates from the late Shang dynasty more than 3,000 years ago, are oracular inscriptions that were carved onto animal bones prior to being burned—once the bones were burned, examining the patterns of how they cracked in the heat and what glyphs were destroyed and/or obscured in that process constituted the divination.
But what is divination? Of course, if you ask twenty different witches this question, then you'll receive twenty different answers. My personal answer is that divination is any use of randomization tools that allows the inferring of abstract meaning from the process and results of randomization as well as the diviner's own intuition.
Which, of course, sounds very technical and wordy. But it boils down to this: the diviner uses a tool or multiple tools (examples: cards, runes, a pendulum, or dice) in a process that involves elements of chance and whose outcome is uncertain. Then, the diviner interprets the results of this process to gain insight.
Notice how I said absolutely nothing about future events there? That's because divination doesn't have to be about discerning future events at all. For me, divination is about getting an outside opinion from unseen external forces (it could be a deity or spirit, or it could be the Universe in general) on a situation.
However, here are also the ethics of divination to consider: is it morally right to consult The Powers That Be and ask the date and manner of someone's death? Is it morally right to tell your querent the answer you receive? Those are questions that diviners ask themselves.