It is called a long s. It looks like ſ in normal type, and ſ in italics.
Long s was part of early roman lettering and in use during the founders time and can be found in many of the founding documents. It fell out of use during the mid 1800's
It is a replacement for lower case s under the following circumstances
ſ is not to be used at the end of a word.
ſ is not to be used before or after the letter f
ſ should not be used before the letter b or k
ſ is used as the first s in any double s (ss -> ſs)
Of note the German ligature ß (eszett) is actually a union of long and short s joined to make a single character for the double ss sound. (ß = ſs)
It is in uſe here as ſort of a mini-revival, and to keep with the blog's antique, founders era feel. Expect to ſee it used more often here.