Arrows used in chemical reactions
The second one, $(2)$, $\ce{&lt;-&gt;}$, indicates resonance structures. 

The difference between the second two is explained in another post - 

What are the correct equilibrium arrows?

Loong basically says that $\ce{&lt;--&gt;}$, $(4)$, indicates forward and reverse elementary steps happening in equilibrium, that is that those are the only two things actually reacting with each other. On the other hand, $\ce{&lt;=&gt;}$, $(3)$, indicates a net reaction, that is that there may be multiple steps in between, leading to this net equation. He does a more in depth explanation of this in his post.

Case $(5)$ indicates a net equilibrium where products are favored, and $(6)$ indicates a net equilibrium where reactants are favored.