virtual memory on compute nodes in a grid

Question

When I run a job on a node, using PBS, and I get finally in the job report:

resources_used.mem=1616024kb
resources_used.vmem=2350176kb
resources_used.walltime=00:06:32

What does the virtual memory really means? I don't think there is a hard drive connected to each node.

Which kind of memory should I take into account when I try to increase the size of the problem, such that I don't hit the 16GB capacity of the node memory, the normal memory (mem) or the virtual memory (vmem) ? Thanks