Is the black wire the hot one or the black wire with a white line down it?

It all depends on where the wire is installed!

In Direct Current systems such as in a car or truck a black wire may be negative but that should not be assumed. Always check first, using a voltmeter.

In Alternating Current mains wiring systems it may be:

• "Hot" in USA or Canada, or other countries which use the same 60 Hz system
  
• "Neutral" in the UK in mains wiring done before 2005. (After 2005 new mains wiring has to use Blue for Neutral, as for the rest of the European Union.)
  
  

IF YOU ARE NOT ALREADY SURE YOU CAN DO SOME ELECTRICAL WORK
SAFELY AND COMPETENTLY
REFER THIS WORK TO QUALIFIED PROFESSIONALS.

If you do any such work yourself,
always turn off the power at the breaker box/fuse panel
BEFORE you attempt to do any work AND
always use a meter or voltage indicator
to insure the circuit is, in fact, de-energized.