Bitcoin (BTC, or XBT on some exchanges)
Bitcoin is THE original cryptocurrency, at least insofar as currently functioning projects go, and functions purely as a cryptocurrency (although many argue that it is more effective as a store of value than as a functional currency). Bitcoin is based upon a famous white paper written by an anonymous developer, Satoshi Nakamoto. Bitcoin became functional in early 2009, when the first block was mined. At this point, Bitcoin is still the dominant project out of thousands of different cryptocurrencies and cryptoasset projects, based upon both market capitalization and merchant acceptance (although as of early 2018, Bitcoin's dominance is diminishing).
Mining/Staking
When the last Bitcoin is eventually mined (many years from now), there will be a total supply of 21 million coins. The present "circulating" supply (not counting coins which may be locked away forever in dead wallets) is several million coins less than that. Bitcoin uses a Proof of Work (mining) consensus mechanism, which gets a lot of criticism because the global electricity consumption of all Bitcoin mining is estimated to exceed the electricity demand of quite a few entire countries. Bitcoin is currently able process a maximum of approximately 7 transactions per second, although that could be improved in the future with second layer solutions such as the Lightning Network.
Development Team
Bitcoin's genesis was brought about by the theoretical work of Satoshi Nakamoto, but notable early developers and enthusiasts included Hal Finney, Gavin Andreson, Jeff Garzik, Mike Hearn, and several more names that are well-known today.
Criticisms, Caveats, and Competitors
Bitcoin has a large number of competitors, simply because it was the first major digital currency project, and a number of more recently developed projects were able to improve upon Bitcoin's limitations with new approaches and concepts. For this reason, a significant number of other cryptocurrencies actually perform various tasks more efficiently. Bitcoin detractors point out to two main issues with the cryptocurrency: The low transaction capacity (which is indirectly causing very high transaction fees due to a backlog of transactions), and the very high global consumption of electricity associated with mining.
Additional Resources
The project's main website: https://bitcoin.org
Link to Wikipedia Page: https://en.wikipedia.org/wiki/Bitcoin
Link to Reddit Community: https://www.reddit.com/r/Bitcoin
Link to Twitter account: https://twitter.com/Bitcoin
Link to Team information: https://bitcoincore.org/en/team
Link to data on Live Coin Watch: https://www.livecoinwatch.com/coins/BTC
Whitepaper: https://www.bitcoin.org/bitcoin.pdf