Dota 2 beginners guide: Items mechanism

Items are the most crucial part in Dota 2 but the question all new players have is "How do they work", don't worry in this short class I will show you all the mechanism and purpose of items in this game.
Chapters:
0:40 What do the numbers mean to the left and right of the heroes mean?
1:10 What is the difference between active and passive abilities?
2:34 What is the health regeneration skill?
3:15 why enemy hero is faster than mine?
4:14 how to steal life in Dota 2
4:56 How do you become invulnerable in Dota 2?
6:13 how to be faster in Dota 2?