I've seen several capitalizations of the term "bitcoin". What do all these mean?
The following answer seems to be the consensus judging from this question on the meta site.
- Use Bitcoin (with a capital "B") when talking about the concept/technology.
- Use bitcoins (no capitalization) when talking about the unit of currency (4 dollars - 4 bitcoins).
- Never use BitCoin or BitCoins
To complete the above consensus, let's add that the “BTC” denomination is a construction by analogy mimicking ISO 4217, a standard published by the International Standards Organization, which delineates currency designators, country codes (alpha and numeric), and references to minor units. E.g. GBP, Pound Sterling, USD, dollar, CHF, Swiss franc, etc.
The word Bitcoin was created by Satoshi Nakamoto when he published his paper. He did not explain any terminology details so there is not an "official version" of how it is spelled.
These variations that you saw were created by different people that sometimes try to distinguish the Bitcoin project from the Bitcoin currency or the Bitcoin client. Which is which can usually be understood by the context of the phrase or by asking the person who wrote it.
There is a consensus that BitCoin is wrong spelling, incompatible with the original Satoshi article. The Wall Street Journal and The Chronicle of Higher Education advocate use of lowercase bitcoin in all cases, and this convention was also accepted by Wikipedia editors.