Tactical nukes are small, a few kilotons yield. They’re delivered by normal weapons systems, such as artillery or strike aircraft over relatively short ranges.
Their job is to break up formations of enemy troops on the battlefield. This can be offensive or defensive. It may seem odd to think of a nuke as a defensive weapon, but the idea is that when you see enemy forces massing to attack you, you drop a tactical nuke on them to break up the attack.
Tactical nuclear weapons are designed to be used near friendly forces or even within friendly territory, for this reason they usually have much lower yield and minimal fallout.
Strategic nukes are the big, long range ballistic missiles. You fire them at high-value targets in the enemy’s homeland (ports, military bases, command and control centres, enemy nuclear missiles, etc). The idea is to break the enemy’s ability to wage war. This may include targeting civilians.
If you would like to see some exact ranges of the blast there is a cool website NUKEMAP that will allow you to enter yields in Kilotons or use some preset weapons and get information like blast radius or estimated casualties.