In Slavic countries it's a courtesy and a greeting to have a kiss on a cheek between a man and a woman, in the past it was common between men aswell but that was a thing from the Soviet era so it pretty much died out already .. and this differs from situation to situation - a in a professional environment a handshake is of course enough, in a friendly or a casual it's mostly a one or two kisses on the cheeks
I'm American, and my best friend's wife is Russian, so when I go visit, her and her friends usually greet me with a hug and a kiss on one or both cheeks. Granted, I've known many of them for years, but even the few that I've only met a few times they've greeted me that way.
I also come from a family of huggers, and we often greet one another with a handshake into a hug, or a hug and kiss on the cheek, so I don't tend to feel uncomfortable when greeting someone who comes from a culture where similar greetings are more common for them than here.
It really does depend on the familiarity we have with the person.
My rule of thumb is that I respect someone's personal space, and default to a handshake, and let them initiate their comfort level of greeting, and, like you said, it differs from situation to situation, so professional and first time meetings handshakes suffice, and in more friendly, casual, or I'm more familiar with the person situations, a hug or kiss on the cheek(s) if they initiate it.
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