Contour lines: plotcontour contourplot
DrwCtour

plotcontour(f(x,y),[x,y]) (or DrwCtour(f(x,y),[x,y]) or
contourplot(f(x,y),[x,y])) draws the contour lines of the surface defined by z = f (x, y) for z = - 10, z = - 8, .., z = 0, z = 2, .., z = 10. You may specify the desired contour lines by a list of values of z given as third argument.
Input :

plotcontour(x^2+y^2,[x=-3..3,y=-3..3],[1,2,3], display=[green,red,black]+[filled$3])

Output :

the graph of the three ellipses x^2-y^2=n for n=1,2,3; the zones between these ellipses are filled with the color green,red or black

Input :

plotcontour(x^2-y^2,[x,y])

Output :

the graph of 11 hyperbolas x^2-y^2=n for n=-10,-8,..10

If you want to draw the surface in 3-d representation, input plotfunc(f(x,y),[x,y]), see 2.2.2):

plotfunc( x^2-y^2,[x,y])

Output :

A 3D representation of z=x^2+y^2