Amateur radio operators are always referred to as Hams, it's basically the most common term for the hobby, at least around here. Where does the term come from? I heard that it's an acronym for something, is that true?
The Etymology of Ham Radio Wikipedia article states that the term was originally used in a derogatory way by professional radiotelegraph operators about amateurs similar to the way an incompetent actor would be called a ham. The term was embraced by amateur radio operators and turned into a term to describe themselves.
I found some other interesting explanations which the Wikipedia article calls false etymologies:
Home Amateur Mechanic magazine as a popular magazine about amateur radio, but there is no evidence that the magazine ever existed.
Hertz-Armstrong-Marconi as the last names of several radio pioneers, but this is again debunked as Armstrong was unknown at the time the term first appeared.
Hammarlund was a company that made amateur radio equipment and their products became such a part of the early pioneering days of amateur radio that they were referred to as "ham products" and people who used them were "ham operators". In reality, the company was a minor and mostly unknown company at the time the term came into use.
These came from an article by n7jy:
Ham was originally a derogatory term short for "ham fisted" meaning the operator couldn't send CW very well. The term "lid" is more common today, I think.
Three members of the Harvard Radio Club (Alber S. Hyman, Bob Almy and Poogie Murray) called their station by their names (HYMAN-ALMY-MURRAY), but it was too long to send with CW so they eventually shortened it to HAM. When the Wireless Regulation bill, which impose regulations on amateur stations such as call signs, frequency allocations, and licensing fees, it was Hyman that testified in front of the committee considering the bill and their little HAM station became the symbol for all amateur stations at the time. (Another source: Origin of the Name "HAM" for Amateur Radio Operators)
A newspaper article written in 1949 about a gathering of radio operators looking to form a club refers to them as hams with the explanation that they were so called because of "the English attempt to pronounce the word amateur".
Originally, the term "ham" was used to describe an operator with poor skills. However, it gained usage pretty early on not only in a neutral but even positive way. The usage of the term seems to have solidified in the second half of the 1910s, as a neutral-to-positive word to describe an amateur (as in unpaid) operator.
Quoting from the Wikipedia article on Etymology of ham radio:
The term "ham operator" was commonly applied by 19th century landline telegraphers to an operator with poor skills. ("Ham" was also in more general use as a slang word meaning "incompetent", as in the phrases "ham handed" and "ham actor".)
as well as
But only a few months [after December 1916], in an indication of the changing use of the term among amateurs, a QST writer uses it in a clearly complimentary manner, saying that a particular 16 year old amateur operator "...is the equal of a ham gaining five years of experience by hard luck."
Today, the term "ham" is used in many English-speaking countries and even some non-English speaking countries to refer to amateur radio licensees.
The relevant Wikipedia sources are given as:
 Brady, Jasper Ewing (1899). Tales of the telegraph. Doubleday & McClure Co. Retrieved February 1, 2013.
 ARRL: Ham Radio History (undated retrieval)
 Early Radio History (undated retrieval)
 "Who's Who in Amateur Wireless", QST, March 1917, p30.
According to Florida Skip Magazine of 1959
It goes like this: The word "HAM" as applied to 1908 was the station CALL of the first amateur wireless stations operated by some amateurs of the Harvard Radio Club. They were ALBERT S. HYMAN, BOB ALMY and POOGIE MURRAY.
At first they called their station "HYMAN-ALMY-MURRAY". Tapping out such a long name in code soon became tiresome and called for a revision. They changed it to "HY-AL-MU," using the first two letters of each of their names. Early in 1901 some confusion resulted between signals from amateur wireless station "HYALMU" and a Mexican ship named "HYALMO." They then decided to use only the first letter of each name, and the station CALL became "HAM."