"The long 's' is derived from the old Roman cursive medial s, which was very similar to an elongated check mark. When the distinction between upper case (capital) and lower case (small) letter-forms became established, towards the end of the eighth century, it developed a more vertical form. At this period it was occasionally used at the end of a word, a practice which quickly died out but was occasionally revived in Italian printing between about 1465 and 1480. The short 's' was also normally used in the combination 'sf', for example in 'ſatisfaction'. In German written in Blackletter, the rules are more complicated: short 's' also appears at the end each word within a compound word.
"The long 's' is subject to confusion with the lower case or minuscule 'f', sometimes even having an 'f'-like nub at its middle, but on the left side only, in various kinds of Roman typeface and in blackletter. There was no nub in its italic typeform, which gave the stroke a descender curling to the left—not possible with the other typeforms mentioned without kerning.
"The nub acquired its form in the blackletter style of writing. What looks like one stroke was actually a wedge pointing downward, whose widest part was at that height (x-height), and capped by a second stroke forming an ascender curling to the right. Those styles of writing and their derivatives in type design had a cross-bar at height of the nub for letters 'f' and 't', as well as 'k'. In Roman type, these disappeared except for the one on the medial 's'.
"The long 's' was used in ligatures in various languages. Three examples were for 'si', 'ss', and 'st', besides the German 'double s' 'ß'.
"Long 's' fell out of use in Roman and italic typography well before the middle of the 19th century; in French the change occurred from about 1780 onwards, in English in the decades before and after 1800, and in the United States around 1820. This may have been spurred by the fact that long 's' looks somewhat like 'f' (in both its Roman and italic forms), whereas short 's' did not have the disadvantage of looking like another letter, making it easier to read correctly, especially for people with vision problems.
"Long 's' survives in German blackletter typefaces. The present-day German 'double s' 'ß' (das Eszett "the ess-zed" or scharfes-ess, the sharp S) is an atrophied ligature form representing either 'ſz' or 'ſs' (see ß for more). Greek also features a normal sigma 'σ' and a special terminal form 'ς', which may have supported the idea of specialized 's' forms. In Renaissance Europe a significant fraction of the literate class was familiar with Greek.The long 's' survives in elongated form, and with an italic-style curled descender, as the integral symbol ∫ used in calculus; Gottfried Wilhelm von Leibniz based the character on the Latin word summa (sum), which he wrote ſumma. This use first appeared publicly in his paper De Geometria, published in Acta Eruditorum of June, 1686, but he had been using it in private manuscripts since at least 1675" (Wikipedia article on Long s, accessed 09-11-2009).
♦ According to R. B. McKerrow, An Introduction to Bibliography for Literary Students (1927), the effective introduction of the reform in England was credited to the printer and publisher John Bell who in his British Theatre of 1791 used the short s throughout. "In London printing the reform was adopted very rapidly, and save in work of an intentionally antiquarian character, we do not find much use of [long] s in the better kind of printing after 1800" (McKerrow p. 309). Though it would be amusing to do so, there seems to be no reason to accept the legend that Bell initiated the change in his edition of Shakespeare because of his dismay at the appearance of the long s in Ariel's song in The Tempest: "Where the bee sucks, there suck I."