In Dummit and Foote´s Abstract Algebra, when talking about the lattice of subgroups of $A_4$, the authors make the statement that, unlike virtaully all groups, $A_4$ has a planar lattice? My question is
What do they mean when they say virtually all groups? Is there a reference for this statement?
On a somewhat different note, are there classes of graphs which can be realized as the subgroup lattice of some group?