Disinformation is incorrect or misleading information which is
intentionally spread with malicious intent. It can be spread
through social media, or any other way that news can spread.
An example of disinformation would be someone writing an
article of false information, and sending it to someone.

The difference between misinformation and disinformation is that
misinformation is the unintentional spread of false information
whereas disinformation is the spread of false information with
intent to misinform.

Misinformation and disinformation are spread through social media
when a post containing false information is repeatedly forwarded,
copied, or reposted. This happens when, say a famous politician
retweets a post of fake news. Then, due to this famous politician
retweeting it, it is spread to all of their followers, and so on.
Artificial Intelligence has been contributing to the spread of
fake news as its algorithm tells it to share things gaining
popularity.

As to explain why people spread false information, people
will do it simply for fun, enjoying the confusion they
create due to posting false info.