Who's asking: E., a potential client
"Em" and "en" are typesetters' references to the space taken up by the letter m and the letter n. An "em" dash takes up two spaces (--) and an "en" dash takes up one (-).
An em dash is what we think of as a dash, and an en dash is a hyphen. Hyphens occur within or between linked words, and do not take a space on either side (e.g., cross-dressing, hara-kiri). Dashes are punctuation marks used to indicate a sharp break, set off a series, provide additional information, or make a side comment. They don't belong in formal writing, and I overuse them myself; a semi-colon, a colon, or a set of parentheses is usually more appropriate.
Traditional typesetting uses no space before or after a dash. AP style, which I prefer, uses spaces before and after a dash. As I told my potential client, many word-processing and publishing programs have their own default settings. What's important is to choose one style, and stick to it.
It's another gray, rainy day here in central Maine. Dad's staying with me for a few days, which makes Dizzy happy; he thinks we should always have company.