It's basically just a large catalog of products, filled by third-parties a-la Amazon now, only it didn't sell anything itself (until recently). Instead, it had detailed characteristics for a lot of products, with corresponding filters in the catalog; and good user reviews. Since Yandex is good at dealing with unstructured text, even poor data exports by vendors end up organized decently on the service. Since Yandex had millions of users on its other services, they all could leave reviews without much hassle. And since Yandex is primarily a search engine, it knows when a bogus review is spammed across the web.
Alas, it's only available in Russian since it works with Russian shops. Every time I need to look for a product on the English web, I lament that there's no service that is quite that solid. Amazon has filters, but search results usually look like simply a bit better Aliexpress. In regard to Google Product Search I don't even know anything particular—I tried to use it a couple times, and my general impression is that it... exists. Not much else.