Steam is Valve's content distribution system. Or in simpler terms, a centralized way to buy games online. Steam filled an almost empty market niche at the right place at the right time, and it has got immensely popular over the years, and has become almost a monopoly on that front. While competing systems have emerged later, they have hard time reaching even a tiny fraction of the popularity of Steam.
In the very beginning Steam was used by Valve exclusively to distribute their own games. (Steam also worked as an anti-piracy system.) However, after some time they opened it to other vendors as well, although at first they gave high priority to third-party games using Valve's own Source game engine. However, they soon started allowing any games to be distributed through Steam without such biases.
Initially Valve had really high standards of quality for what they would and wouldn't accept to be distributed through Steam. On the plus side this ensured that most if not all games bought from Steam were technically of decent quality. On the minus side many companies and developers, especially indie developers, found it almost impossible to distribute anything through Steam, while well-established big game companies had almost automatic approval. Sometimes this meant that great indie games were rejected.
Valve has since loosened their acceptance policies. The problem with this? Valve has gone to the absolute other extreme: Now they are basically approving anything that either gets enough votes in their greenlight system, or comes from a known game company. There seems to be no quality control at all. In other words, while in the beginning the standards of quality were almost draconian, now there is no quality control whatsoever.
This means that there are tons and tons of garbage on Steam right now. There are indie games of absolutely atrocious quality that cannot even be called games at all. Many people get duped into buying this garbage because they assume that if it's published on Steam, it has to be at least playable at some level.
Another phenomenon that has surfaced in recent years is that many companies are dumping to Steam quite old games they own the intellectual property, for a quick buck. In many cases they may even masquerade the game as being a new game, ie. published recently, and may even make the screenshots deceptive in this sense. In other words, a game you bought that you may think was recently published (because the publication date on Steam was like 2013 or such) may well be a game first published in 2002 or something.
This is not always bad, of course. In some cases this gives a great opportunity to get older games that were extremely good, and which would otherwise be basically impossible to get. The problem is that many of these old games were of atrocious quality (or very mediocre at best). It outright smells of fraud when the publisher does not clearly express that it's a 15 or so years old game, but tries to masquerade it as a new game.
And of course in many cases the publishing company made the minimal possible effort to have the game running on current computers. This often means that the game might not even run properly on some PCs, or have other kinds of problems.
This is becoming more and more common because it seems that Valve has stopped quality control completely. They don't seem to even check if the game even runs at all in modern PCs.
And of course Steam does not offer refunds, period.