what exactly is the difference between invasion defense and just defense?

Invasion defense is only used when the planet is being invaded. While the planet is under normal attacks (where the population is being killed), it will not have any effect.

what does invasion defense do? since it wont be in effect when the planet is being attacked.

When the planet population drops very low, the planet can then be invaded. This can only be attempted once per day per ship. This is a one-time attempt, like hacking or raiding, that can succeed or fail. Having invasion defense during this time will lower the chances that the planet will be invaded.

by how much does it lowers the rate of success?

Invasion defense lowers the invasion potential at the same rate as standard defense.

If you fail an invasion, it will inform you of the % chance you had of success.

Also wondering how does attacks and defense actually works. will one with 100 attacks be able to penetrate through another with a 100 defense? vice versa. Lets say i have 50 invasion defense , how much will it lower the chance of invasion by others, does it has anything to do with the opponents attacks when invading as well?

A ship with 100 attack and a planet with 100 defense will roughly result in a 50% chance for the invasion to be successful. A planet's attack and defense both contribute to this chance, while only a ship's attack and invade abilities will.

There should be a like genocide option (Which that is probably a bad wait to put it but it sums it up >_<) where we can just wipe the people off the planet or like a "raze" option where I can destroy all the structures on an enemy planet after i've defeated everyone on the planet. Would be fun xD.

Haha, and Implode Planet option? Would be kind of cool, could wipe it off everyones planet list, and put it back in circulation as a virgin planet again.