Camel case
This is a convention for naming identifiers. Identifiers start with a lower case letter and words are separated by capital letters rather than underscores. So, for example, one would use someMultiWordName rather than some_multi_word_name.
See also Studly capitals.
It is an open question on how to name acronyms using this scheme. Some prefer editSGMLDocument while others prefer editSgmlDocument. The former is arguably more accurate and certainly more popular, but can get ugly. It's also not obvious how to name an identifier which starts with an acronym.
For these reasons, User:AndrewBromage personally prefers to treat acronyms as words.