What is Peercoin?
Peercoin is a digital currency similar to Bitcoin but with a key differences. Peercoin is based on an August 2012 paper which listed the authors as Scott Nadal and Sunny King. Sunny King, who also created Primecoin, is a pseudonym.

Differences between Peercoin and Bitcoin
Peercoin was inspired by Bitcoin, and it shares much of the source code and technical implementation of Bitcoin. Peercoin's major distinguishing feature is that it uses proof-of-stake/proof-of-work hybrid system.

Energy Efficiency
Most cryptocoin mining requires specialized hardware, but Peercoin minting can be done on any computer. Minting is energy-efficient, because it is based on the Peercoins you hold, rather than on your processing power.
Amount of coins
Peercoin does not have a hard limit on the number of possible coins, but is designed to eventually attain an annual inflation rate of 1%.
Different hashing algorithm
Peercoin's proof-of-stake system was developed to address the high energy  consumption of Bitcoin. The proof-of-stake system was designed to address vulnerabilities that could occur in a pure proof-of-work system. With Bitcoin, for example, there is a risk of attacks resulting from a monopoly on mining share. As Proof-of-stake becomes the primary source of coin generation, energy consumption (relative to market cap) decreases over time.