Staking is the process of investing in a project which is built on the PoS~Proof of Stake algorithm. Your coins are locked on their specified wallet for a period of time and you earn a percentage of your holdings in time intervals. Proof of stake is an alternative to the Proof of work algorithm used by the Bitcoin network to reward miners.
Staking is similar to mining on the Bitcoin network
My most important question: Which is better, staking or investing?
Staking is a form of investing, you'd need to specify what form of investment to compare it with.