What is the differences between Lead Vector and Cardiac Vector? How is Lead Vector being used to measure ECG Signal?

It is a bit tricky to grasp these differences from a simple text – I highly recommend this web site which is sort of an introduction to the basic concepts of ECG.

But basically a 3-D "cardiac vector" at a certain instant, is projected to a plane and forms a 2-D (plane) vector. The 2-D vector is further projected to a linear lead, and forms a 1-D vector. The size of the 1-D vector is the voltage, which is represented by the deflection of the electrocardiograph needle; there are 12 leads with different positioning.