1. What is meant by junk science?
Junk Science is typically used as a weapon against gullible people to obtain recognition and/or compensation for something that is not valid. Typically the media, politicians, lawyers and even government officials will use unfounded and skewed data to sensationalize a particular scenario to push an agenda for some ulterior motive.
For example, a Pharmaceutical company might try to say that a particular disease is caused by say not covering your ears in the wind. They might take this information from data that shows some slightly significant correlation between working in windy conditions and getting ear infections more than 3 times a year. The junk science perpetrators thrive on sensational headlines and finding new ways to trick people with "data" that cannot be replicated nor explained by peer review. It is essentially taking data and finding correlations that do not actually exist or completely fabricating data to support a claim.
2. What is meant by scientific fraud?
Scientific fraud is outright faking or fabricating results. It could be using improper techniques in the lab to using extreme bias when selecting test subjects for a study. It is essentially done by scientists or corporations paying researchers to use faulty data or practices. Junk science usually involves a lot of scientific fraud.
3. Are the first two different from mistakes in science? How?
Junk science and scientific fraud are certainly different from mistakes in science. Mistakes in science are not dubious. Now, several people might pretend that they made a mistake if the general public started to realize that data was misrepresented, and proving that it was intentional might be difficult. However, it doesn't mean they are similar. No, there is a big difference to accidently mixing a chemical and forming a new substance that has some unique and beneficial property, than