Your reference gives a very good summary of what those categories mean. Basically ‘Real mem’ is the amount of RAM reserved for that process, and is split into ‘Private’ (used solely by the process and not accessible to others), and ‘Shared’ (can be used by other processes, e.g. which might need the same library of functions). ‘Virtual mem’ is the total amount of memory reserved for the process, and is not limited to RAM (e.g. some processes offload data to the hard disk when not much RAM available).