Armor+Minus Armor

Hi!
I am playing Dota for a good while now but I still don't rly get how armor works.
I know that there are armor values to be avoided ( eg. -7-7) but I don't really get what is right and what is wrong.
Some say that the more armor you have the less effective more armor will be and some keep telling you that there is no difference at all.Help!

http://dota2.gamepedia.com/armor?cookieSetup=true#Damage_multipliers_at_different_armor_values

on low armor levels getting armor makes more difference. for instance from 2 to 3 is a bigger difference then 15 to 16.

i think that above 20 armor it doesnt make a difference anymore, exept for countering minus armor. for instance if you have 30 armor and get hit by -10 you are still at max.

http://dota2.gamepedia.com/File:Dota_2_armor_graph_plain.svg

This graph is all you'll ever need to know about armor.

It shows the damage multiplier in function of the armor value.

You see that the slope of the curve is very steep around zero and very flat far from zero. This means that armor changes (both + armor and - Armor) are at their most effective when the armor value is around zero.

So for example your Desolator will be extremely effective against the enemy Kunkka sitting at 6 armor, but almost useless against the enemy Dragon Knight sitting at 25 armor. The whole point of minus armor is to bring the enemy armor value to zero, or even better slightly below zero. If you do that they'll melt in a few seconds. Otherwise don't bother since damage items are better.
Also, armor reduction comboes really well with itself. That's why you commonly see Desolator picked up on heroes like Templar Assassin or Weaver, since added to their natural armor reduction skills it can bring enemy armor below zero.

For plus armor, the more HP you have, the more armor you need for your HP/armor ratio to be effective. In the late game if your HP value is high then building armor is always better than building HP.

ty so much