What are tactical nuclear weapons and how might Putin use them?

Russian President Vladimir Putin said this month he's not bluffing about using nuclear weapons. If he were to follow through on that threat, it's likely his weapon of choice would be tactical nuclear weapons. And he's not the only one talking about using them. North Korea's recent missile tests involved "tactical nuclear" drills to simulate hitting the South, according to the North's state-run media. 

Tactical nuclear weapons are sometimes referred to as "small nukes," even though they still cause devastating fatalities and destruction. They are designed for limited strikes against relatively close military targets, like command posts, instead of destroying cities from far away.