I could go to a lot of detail, but the short form is they are bombs of less than about 30 kilotons, most under 20.

In terms of power, they're equivalent to the bombs dropped on Japan at the end of WW2, but modern ones are much smaller.

Whether or not they still have them is debated, but at one time the USSR definitely had 152 mm nuclear artillery shells. They maxed out at about 20 kilotons, and the guns that can fire those can hit out to about 45 kilometers.

Some of those guns are in the Ukraine theatre. This shit is very scary.