So I think that this has mostly been said across the replies thus far but to compile things in one place:

First there is the question of terminology - a "loan shark" is a term used primarily to describe a criminal moneylender who's running a scam - they'll lend you money, but then any money you pay back will go towards absurdly high interest rates, and not pay down the loan balance; and if you fail to pay, they get violent (or have "associates" who get violent on their behalf).

However, because humans love hyperbole, the term "loan shark" may be used to describe payday loan lenders - essentially the last legal resort for people who need cash - the idea is that if you borrow money from them, you don't need a credit check or anything like that, and they give you some high-ish percentage in a lump sum of what you would get on your next payday, in return for your next paycheck in its entirety (ex: if you make $500 per paycheck, you might get anywhere from $450-480 from a payday lender) - the problem comes up when you can't pay your whole paycheck to them on time, and they let you pay it off over a longer term. The way these usually end up structured, you could pay thousands or tens of thousands in little $15 payments over years for that $450 you borrowed, if you don't quickly pay off the principle. And if you default on this loan, a lot of places tend to employ debt collectors who are a bit less keen to follow the law than you'd hope, and will make it seem, at least, like they might actually commit violence or do something within the law to make your life miserable.

The reasons that this is bad are many, but the main one is that if you are in a situation where you need this type of loan, it is almost certainly because you need some other sort of help instead, but the money issue looks like the biggest issue. Even if it is a strict money issue, this is a bandaid on a bullet wound and you need financial planning help.

The fact is that even the legal companies are predatory - the real money in payday lending is in those absurd APRs, and knowing that most people feel an obligation to pay their debts even if those terms are grossly unfair; people tend to not know their rights, tend to be pretty bad at math that involves percentages, and all of this helps to rig the game in their favor.

Now, the very tiny "but" - for the people who have absolutely no other option, this can be the lesser of two really shitty outcomes. And this is one thing I think people get wrong talking about them - people aren't stupid, they know it's a bad deal most of the time, but it's a less punishing option than defaulting on a credit card or car note or something else like that.

But by and large, if you have any other option, you should take it over a payday loan. Borrow from friends and family, see if you can take out a personal loan from the bank, ask for an advance from your job, angle for a raise, take on a part time second job for a while to pay off any of the above and even things out, skip a few meals. These will all hurt much less in the long run than the giant barbed financial probe that are these predatory loans.