Lake effect snow is when a cold air mass (from up north in Canada) comes down and swoops across a warmer body of water like Lake Erie.
The difference in temperature between the air mass and water mass create massive convection - lifting moisture off the lake and as a result, the cooler air chills that moisture into a massive snow machine.
Once Lake Erie freezes over during the winter, the lake effect snow declines.
Lake Ontario typically never completely freezes over because of its depth. The upper layer of water cools - and when storms whip winds across Lake Ontario - the water table inverts... where the warmer water from much lower rises to the surface and the cold water sinks to the lower depths. This is why Lake Ontario tends to not freeze... therefore, you get lake effect snows most of the winter when the wind is just right.