Dogecoin is a peer-to-peer, decentralized digital currency, as well as the name of the network it transacts on. You'll often see Dogecoin, the currency, referred to with its ticker symbol, DOGE.
Dogecoin started as a joke based on a popular 2013 meme. However, its community appeal, launch mechanism and utility for tipping social network users quickly saw it build a significant following.
The network is actually a fork of the now-defunct Luckycoin. Luckycoin, itself a fork of Litecoin, had a random block reward, ranging between 1 and 1 million coins. Dogecoin borrowed this model, only it halved the maximum random reward every 100,000 blocks. In 2014, the block reward structure was changed to issue a fixed but steadily decreasing number of DOGE each year. Another change in 2015 set the reward at 10,000 DOGE per block.
DOGE uses the same Scrypt proof-of-work algorithm as Litecoin. However, its initially inconsistent block rewards made it unpopular with miners. Sensing that Dogecoin's low hash rate made it a likely target for a 51% attack, Litecoin founder Charlie Lee worked with Dogecoin's founder to introduce merge mining to the network. Doing so enhanced Dogecoin security and allowed Litecoin miners to simultaneously mine LTC and DOGE with no additional energy use.