In many cases a verb x has a corresponding noun (often formed by adding "-er" or "-or" to the verb) meaning "one who does x." For instance, the verb "to spoil" corresponds to the noun "spoiler," meaning "one who spoils." Thus, it seems reasonable to assume that to find out what a "burglar" does, one would remove the "-ar" suffix, resulting in the verb "to burgle." But as it turns out, the word "burglar" was around (in English) for three hundred years before someone coined the verb "to burgle" by subtracting that alleged suffix. (On the other hand, those who claim that therefore "burgle" isn't good English may be unaware that the first use of that verb predates the first use of "burglarize" by a year.)
Any time you remove a piece that you incorrectly believe to be a prefix or suffix from a word, and thereby create a new word, the result is called a back-formation. (The process of creating legislation is called "legislating"; the process of creating regulation is called "regulating"; so I've long tried to convince people that the process of creating a back-formation should be called "back-formating." Alas, this term hasn't caught on, perhaps because the dictionary says that this process is known as simply "back-formation." I feel that this is narrow-minded and uncreative of the dictionary, but nobody asked me.) If one were to state that a "cle" is whatever an "uncle" is not, or that to "register" is to gister more than once, or that a "list" is one who ls, one might be engaging in back-formation. Then again, one might just be engaging in silliness.
Some time back, Dominus suggested a puzzle along related lines. He pointed out that in some cases a verb and its corresponding "one who does x" noun are the same: "A person who cooks is a 'cook,' not a 'cooker'; a person who guards is a 'guard,' not a 'guarder.'" The puzzle is to come up with another example in which a person who does x is called an x, not an x-er. If any of you come up with answers, please send them in. As Dominus adds, "Near-misses are also interesting." (So, is someone who back-formates called a back-formater?)
The inverse of this puzzle is also somewhat interesting. Given that a reader is one who reads, and a baker is one who bakes, what is a plumber? One who plumbs, I suppose, though that sounds a bit odd. And a butcher is not one who butches, but one who butchers (which incidentally provides one answer to Dominus' quiz). Going further afield, a looker is not generally one who looks, but rather one who is looked at. And what about a grocer? More generally, are there many "-er" nouns that describe an occupation or profession but can't be truncated to produce a corresponding verb (and have not been back-formated in the way "burglar" has)?
By the way, I note that a baker works in a bakery, and a grocer works in a grocery. A gardener, though, works in a garden, not a gardeny; and a writer doesn't work in a writery. So who works in a rookery?