When typographers talk about gutters, they are not referring to to channels or troughs on roofs used to direct water, but they are referring to a similar concept. Typographic gutters are the white spaces between two pages of a book, or more generally, between columns of text. They are a type of margin, and arranging them properly is a crucial element of typography.
Poorly spaced gutters can make text difficult to follow or read. In the case of those between the pages of a book, those that are badly designed can actually obscure some of the text. In this instance, if the space is not made wide enough to allow for the binding of the book, the text will run into the seam of the binding, rendering it difficult or impossible to read. Gutters that are less closely spaced can still pose problems, and arranging them is actually a fine art.
Much like rain gutters, those used in typography are designed to control and direct something, in this case, the attention of the reader. A solid block of text without a break can be extremely difficult to read and comprehend. As a result, typography integrates the use of gutters, margins, and other white spaces to make the surface visually interesting. The breaks provided by white space help to direct the eye, keeping the reader on task. When a reader sees columns of text separated with white space, his or her training suggests that the columns should be read individually.
A newspaper provides an excellent example of gutters and margins. When a reader looks at the front page of a newspaper, he or she will notice that the text is broken up into an assortment of columns, clearly separated by white space. These white spaces they inform the eye that the pieces of text should be read independently and sequentially. Each column is a story, and each gutter is like a chapter divider. They also provide a rest for the eye, allowing the reader to focus and absorb the material. A newspaper with a front page that took the form of a solid block of text with no interruption would be difficult to read.
Gutters are slightly different than margins. The term “margin” is used to refer to any sort of white space, typographically, usually on the edge or margin of printed material. Margins are important, because they break up images and text on the page. Gutters are usually used specifically to talk about the space between columns of text. That space could also be considered a margin, so it could be said a gutter is a margin, but not all margins are gutters.