Let's use the plots package 

We'll begin by investigating the following function f 

Now plot this 

Now use a countourplot(..) command to plot contours 

The above uses a default number of contour levels (8). We can also set contour levels to values we select. Here we are selecting contour levels corresponding to f(x,y) = c where c is one of the list of points. 

Now to get level surfaces in 3 dimensions, we use the implicitplot3d(..) command (contourplot3d does not do level surfaces). Recall that the implicitplot3d will plot a function defined as - f(x,y,z) = c - which is simply a level surface. 

Here we plot 3 level surfaces corresponding to f(x,y) = 0, f(x,y) = 10 and f(x,y) = 20