Monsters have three types of defense, both reducing in some way the amount of damage they receive. The first type is Normal Defense, used to reduce the damage of the bows. There is also magic defense, reducing the tower and spell damage. Lastly some monsters have some Crowd Control defense.
As said, magic defense reduces any damage that is related to an specific element.
First of all, this kind of defense reduces the damage done by towers. Testing shows that the defense reduces a % equal to its value. For example a defense of 20 will reduce the damage the monsters take by 20%, therefore, only 80% of the towers damage will be applied to the monsters.
Additionally, magic defense reduces the damge of the same spell type by half. This kind of reduction is not affected but how many points of defense there are but wether or not there is any.
This defense can be seen at the top side of the health bar, via a symbol. If the symbol doesn't appear, then this type of resistance isn't active.
Monsters also have another defense that isnt bound to an specific element. This defense reduces the damage taken by a % equal to its value.
Crowd Control Defense
Apparently, monsters also have some resistance to being crowd controled by skills. This grows progressively throughout the game starting at their same skill and following onto the next. I.e, earth monsters will start being less affected by the CC done by earth spells, then water spells...
Also, bosses are not able to be stunned (I don't know about slows), but mini bosses will be stunned/slowed.