Level Sets

Level curves for functions of two variables: f(x,y) = c

f[x_,y_]=x^2-y^2;
ContourPlot[f[x,y],{x,-5,5},{y,-5,5}]

g[x_,y_]=x^2+y^2;
ContourPlot[g[x,y],{x,-5,5},{y,-5,5}]

h[x_,y_]=2-6x+2y;
ContourPlot[h[x,y],{x,-5,5},{y,-5,5}]

??ContourPlot

Level surfaces for functions of three variables: f(x,y,z) = c

F[x_,y_,z_]=(x/2)^2+(y/3)^2+(z/5)^2;
ContourPlot3D[F[x,y,z],{x,-10,10},{y,-10,10},{z,-10,10}]

F[x_,y_,z_]=(x/2)^2+(y/3)^2+(z/5)^2;
c=4;
ContourPlot3D[F[x,y,z]==c,{x,-10,10},{y,-10,10},{z,-10,10}]

F[x_,y_,z_]=(x/2)^2+(y/3)^2+(z/5)^2;
Animate[ContourPlot3D[F[x,y,z]==c,{x,-10,10},{y,-10,10},{z,-10,10}],{c,1,4}]