Four wire O2 sensor source car?

[LotusM50 at aol.com]

In a message dated 97-05-22 13:46:23 EDT, cloud at peaches.ph.utexas.edu (T.
Cloud) writes:

<< >Ford cars had "allways" heated oxygen sensors, as many japaneses 
 >cars. 
 
 don't know about Ford cars, but some early Ford trucks ('80's)
 used 2-wire sensors >>Excuse my ignorance on this.  But what is the
difference betwen a 4 wire and a 3 wire heated oxygen sensor...