> with(plots):

In the following, I use "implicitplot3d" to find the surfaces
described by the equations I use.  I need to start with the command
"with(plots);" or maple won't understand the command "implicitplot3d".

This describes a cylinder of radius 3.
> implicitplot3d( x^2 + y^2 = 3^2,x=-5..5,y=-5..5,z=-5..5);

We plot r = 3 in _cylindrical_ coordinates.  This gives a cylinder of radius 3
> 
> implicitplot3d( r = 3, r=0..5,theta = 0..2*Pi, z=-3..3, coords=cylindrical);

This gives a cylinder of radius 3, cut down the center.  Note that it
doesn't really look like a semicircle when you rotate it --- this is
because Matlab scales things a little odd.
> implicitplot3d( r = 3, r=0..5,theta = 0..Pi, z=-3..3, coords=cylindrical);

This describes a _sphere_ of radius 3
> implicitplot3d(r=3,r=0..4,theta=0..2*Pi,phi=0..Pi,coords=spherical);

This describes one quarter of a sphere of radius 3
> implicitplot3d(r=3,r=0..4,theta=0..Pi,phi=0..Pi/2,coords=spherical);