"Jarl" is a noble title, a rank below that of a Konungur (king). It is etymologically equivalent to the Old English "Earl". A jarl owes service to a king, and is served in turn by his thanes (see Thane), particularly his household guard, known as huskarlar (see Huskarl)
The proper plural should be jarlar, but I often use an Anglicized "jarls" for clarity.
Equivalent: Earl, Scandinavian Jarl