In a lot of countries outside the United States there are similar outdoor markets where goods and food are sold right on the street. The rionik is just an outdoor market where lots of people come together to buy what they need. Rioniks in Russia vary in size and can serve a very specific purpose, such as only have shoes or boots, or can be a smorgasbord of anything and everything (I like to call these your outdoor Walmarts).
Most cities have one to two major rioniks (Moscow of course has lots that I don't know where to start counting), and are an exciting place to shop. The rioniks are open daily from about 7:30 until about 4:30. The rionik closes for a couple of holidays during the year and when the weather drops to -35 and below. During a recent trip to the rionik we purchased some food (halva for me), socks, and sunglasses. We checked out other clothing and looked for nailclippers as well but didn't buy any.
If you are looking for something of nice quality then the rionik isn't the place for you (I call it the outdoor Walmart for a good reason). However, if you are looking for a good deal then the rionik is definitely the place to go. You can usually get prices lowered down if you try to (however I have found that for some reason most natives don't try...Americans always do though). Maybe that is one of the reasons that I love the rionik so much is because if you want a sweet deal, then you can make it.
For example, when I was in Moscow a few years ago I was looking for some Matroshki dolls. Since Moscow is the best place to buy the dolls on the rionik or at the street vendors we started walking around. After visiting about 10 stands and having each one make some kind of offer, I finally made my final purchase of 10 matroshki for about $90. Considering the starting offer of $250 that they were asking for, I felt pretty good and I could probably sell the dolls for a pretty penny in the United States for them if I wanted.
One thing we never did though was to buy meat at the rionik. Let's just say that it looked scary enough that I never do. However, that's not to say that I don't eat the meat from the rionik because where I am staying now they normally buy their meat from there and so far I haven't gotten sick (keep my fingers crossed). Vegetables are super cheap during the summer season and you can usually get a kilogram of tomatoes for under $1 (that's under .50 cents a pound for fresh ripe tomatoes).
Overall the rionik is a fun atmosphere and if you visit on the weekend then expect the crowd. If you plan on visiting Russia though, definitely make some time to visit a rionik while you're in the country in order to get a good taste of Russia.
Another video on the rionik.