Northern Soul has unfortunately gained a reputation for becoming a trainspotting type activity - that the music is now taking second place to the obscurity of the artists and the labels on which they recorded. The following is a definition of Northern Soul which is probably as accurate as any:

Northern Soul is a type of mid-tempo and uptempo heavy-beat soul music (of mainly African American origin) that was popularized in northern England in the mid 1960s. The term also refers to the associated dance styles and fashions that emanated from the Twisted Wheel club in Manchester and spread to other dancehalls and nightclubs; such as the Golden Torch (in Stoke-upon-Trent), the Blackpool Mecca, and (in 1973) the Wigan Casino. Northern soul dancing was usually athletic, resembling the later dance styles of disco and break dancing. Featuring spins, flips, and backdrops, the northern soul dancing style was inspired by the stage performances of visiting American soul acts such as Little Anthony & The Imperials and Jackie Wilson.
(courtesy of www.answers.com - Northern Soul definition)

Some aficionados have claimed that it was a spin-off mainly from Wigan Casino:

Northern Soul

Our own position is that Soul is Soul and that the truly great artists will remain popular long after the more obscure but often derivative rarities have become nothing more than a footnote in the history of the African American soul heritage which peaked probably circa 1967. The true test for us is: when we hear a track, does it transport us back to the heady excitement of a Twisted Wheel all nighter and produce the adrenalin rush that we experience then, once more. If so, it passes our test. The last track to do this for me was suddenly hearing Junior Walker and The All Stars - I'm A Roadrunner.