If you are asking, what is a ligature, check out this web page on OpenType and advanced typography. Wikipedia also has a fairly decent entry. Interestingly, I have now been to two custom stationery stores in New York City. Neither store knew what I was talking about when I asked if their letterhead used ligatures.
Incidentally, this is not a new concept with respect to computers and typography. TeX has had support for ligatures since its creation in the late 1970s.