About two and a half billion years ago, the Earth finally cooled down enough for it to snow for the first time. Most of the time since then, the Earth has been warm enough that it didn’t snow at all. But from time to time, the Earth becomes cooler, entering an Ice Age. We are in one of these Ice Ages now, though not at the coldest part of an Ice Age, and so it snows sometimes, in some places. But it doesn’t snow as much as it would in the coldest part of an Ice Age.
Snowflakes, because they are made of ice, are really little crystals.
Snow usually falls either in the winter (if you are in the Northern Hemisphere) or in the summer (if you are in the Southern Hemisphere) or on top of high mountains, where snow often lasts all year round. Near the Equator, it doesn’t snow at all, except on the very highest mountains.