I know some basic commands only, and also don't know what is the usage between the three types of command blocks.
Impulse Command Blocks are regular blocks for simple commands
Chain Command Blocks are for difficult /crazy commands that need more than one command block.Chain Command Blocks next to each other make awesome things!!
Repeat Command Blocks are for simple comamnds (as impulse) and it will repeat the command you put 23 or 25 (not sure) times..
As in the picture with lightnings.
Just put in the repeat command block summon lightningbolt , turn a lever on aaand BOOM TORNADOOOO
XD
I'll need command blocks due to the fact you teleport in some games