Differential = partial derivatives?

[Mimmo]

What's the differential!!??
http://en.wikipedia.org/wiki/Differenti ... ematics%29

f(x, y, z) = 2x + 3y + 4z
Then what's the differential in (x0, y0, z0)?? Is it the partial derivatives summed? :?

 

[robbwrr]

df = (f_x) dx +( f_y) dy +( f_z) dz