Difference between 'Void Ab Initio' and 'Voidable'

Maine pehle bhi kuch suna hai iske bare mein, lekin abhi tak samajh nahin aai. Kya yeh dono hi ek hi cheez hain ya alag-alag? Mainne pehle se hi 'Voidable' ka matlab samajh liya hai, jahan ek agreement ya contract ko invalid hone se bachaya ja sakta hai, lekin 'Void Ab Initio' ke baare mein koi pata nahin. Kya yeh bhi ek agreement ko invalid karta hai, lekin phir yeh kitna alag hai? Maine koi example ya case nahin dekha hai, jahan is cheez ka use kiya gaya ho. Help, please! Ye sab theory hai, but real-life examples chaahiye.