The German(-cum-English) word Führer came into English in the mid-1930's, with a meaning synonymous with 'Adolf Hitler', (or at least, 'Hitler-esque leader'), from the fact that Hitler himself used it in his title.1 Its German meaning, though, straightforwardly, is simply "leader".
The complete title he adopted was actually Führer und Reichskanzler, but the Reichskanzler part never made it into English. Hitler was apparently inspired to take a title meaning 'leader', by the fact that Italian dictator Benito Mussolini, referred to himself as Il Duce, meaning the 'Duke' or 'leader'.
In modern German, the term Führer has preserved its pre-World-War-II innocuous meaning, referring specifically to military 'captains', and more generally, to any leader.
The word's etymology is particularly interesting, though. In German, it can be traced to the verb vüeren, meaning "to lead", which in turn traces to the Old High German fuoren, which, in addition to meaning "to lead", also carries the sense of "setting in motion".
But hang on—the etymology is even more interesting than that. An ultimate Indo-European root of Führer (and vüeren, fuoren, and even several Germanic words in English, like fare and far), is the prefix per-. This Indo-European per- is extraordinarily common in Latin and English, because, as the American Heritage Dictionary of Indo-European Roots describes it,
[per-- is the] [b]ase of prepositions and preverbs with the basic meanings of "forward," "through," and a wide range of extended senses such as "in front of," "before," "early," "first," "chief," "toward," "against," "near," "at," and "around."
Interestingly (and ironically), this makes the English words paradise, priest, and pristine distant (but demonstrable) cousins of the word Führer.
One final note: when writing in English, it is common and entirely permissible to eliminate the umlaut, and simply write fuhrer. It is, however, usually capitalized, but not because it is a noun (as all German nouns are capitalized). Rather, it is usually capitalized in English because it is a proper noun.
Sources & Notes
1.mauler says . . . Hitler actually began to be called [Führer] by his followers in the NSDAP around 1922.
2. The American Heritage Dictionary of Indo-European Roots