We're not talking about those parking spots in old-timey towns in America. We're talking about that 'slash' character (/) found on most keyboards to the right of the period. It's also known as virgule, solidus, slant, or forward slash, to distinguish it from a backward slash, or backslash (\).
Use the diagonal only when it has clear advantages over alternatives; consider rephrasing to avoid it.
Probably the most common use of the diagonal in general writing is in the word and/or -- if it really is a word rather than a convenient device to save writers trouble and suggest that they have gone to the trouble of considering every possibility. Generally the diagonal has something of the effect of the word and or the word or, and since neither and and or nor and or or is acceptable, we have and/or. Often when and/or is used, it can be replaced by and or by or or by a rephrasing that takes only a few more words -- this or that or both rather than this and/or that. And/or can be effectively used, but too often it merely camouflages muddy thinking.
Sometimes the diagonal replaces the hyphen in compound nouns, and because it does not just join words but suggests that they have equal value, its effect is slightly different from that of the hyphen. It has a definite advantage over the hyphen in some compound nouns in which one or both elements of the compound are already compounds: treasurer/director of sales, senior vice president/director of sales. However, it usually has no advantage over the word and in such compounds.
As a mark of punctuation, the diagonal is not as firmly established in the language as the hyphen. There is something nonliterary about it; it seems more appropriate to summaries, notes, technical material, and other such forms of writing than to formal prose.
The diagonal can also indicate per or divided by, as in 50 miles/hour and price/earnings ratio. Uses such as the first example would be appropriate only in compact technical prose. Price/earnings ratio is common in general writing on financial subjects, but the phrase is also commonly hyphenated.
The diagonal has some special uses that go beyond punctuation. For example, it is used to indicate line breaks in poetry that is run in with prose
I have forgot much, Cynara! Gone with the wind, / Flung roses, roses riotously with the throng, / Dancing, to put thy pale, lost lilies out of mind;
Today's sources: The Handbook of Good English, by Edward D. Johnson, Chicago Manual of Style, and the Web.