The North Circular isn't circular. It is a semi-circular road which goes through the North half of London. It isn't as far out as the M25 To the West it stops at the Chiswick Roundabout (where it continues as the A205, South Circular Road). To the North it goes past Ikea in [Wembley]? and through Brent Cross Shopping Centre. To the East it reaches Ilford and finishes at the [Woolwich]? [Ferry]? terminus.
Generally if you need to drive from one side of North London to another you should consider using the North Circular since it has been upgraded over the years with more lanes than most non motorway roads. It is usually faster than going in a straight line. Sometimes however it is worth considering going straight out to the M25 and around the long way.
The North Circular is a better road than the [South Circular]? which varies from three lanes each way, to one lane each way in some sections. Also, the North Circular Road bears a single number, A406, for its whole length. Although much of the South Circular is the A205, other parts of it overlap other routes, including the A3.