What is this type of hat with small pom-poms at the sides called?

The traditional Russian terminology is "Ushanka", though it is often referred to in English as a "trooper hat".

An ushanka (Russian: уша́нка; IPA: [ʊˈʂankə], lit. "ear hat"), also called a ushanka-hat (Russian: ша́пка-уша́нка; IPA: [ˈʂapkə ʊˈʂankə]), formovka (формóвка), trooper hat, is a Russian fur cap with ear flaps that can be tied up to the crown of the cap, or fastened at the chin to protect the ears, jaw and lower chin from the cold. The dense fur also offers some protection against blunt impacts to the head.

The word ushanka derives from ushi (у́ши), "ears" in Russian.

Another option is "trapper hats", which seems to describe the same style of hat.

Here's an example: