So there are various types of Proof of STAKE , and amongst others, these two ....
Age based selection (RDD) or
Randomised (PiVX)
Age based - need a lot of coins and to 'stake' it for a long time to get your rewards - so people with more money who buy more get a bigger piece of the pie.
Randomised - a little fairer. you don't have to hold as many coins, and you have an equal chance to get coins....
Overall POS Weakness -
People hold it and maintain it in wallets to get rewards = less liquidity
There's also Delegated POS (DPOS) and Consensus which is Poor of Activity (POA - currently only Decred has this) .... but those need more explaining!
So.... basically staking is like 'dividends' and in some way it makes the coin more stable cos people 'hold' the coins. Hold the coins, earn some rewards, and hope they rise in value over all.
I think that made sense ....let me know if it didn't.
Here's trying to figure it all out ....
Completely Clueless
See more here ( and the image came from here too): https://www.google.co.uk/url?sa=i&rct=j&q=&esrc=s&source=images&cd=&cad=rja&uact=8&ved=0ahUKEwih-J-09ubZAhWCC8AKHZEbDTkQjRwIBg&url=https%3A%2F%2Fsteemit.com%2Fcrypto%2F%40miguelalonso1%2Fwhat-is-the-mining-pos-proof-of-stake&psig=AOvVaw3wQCIHenFhcZXA_ynLnbOd&ust=1520948878668384