Different memory types in System Monitor

On the ubuntu system monitor there are different types of memory listed: Virtual Memory, Resident Memory, Writeable Memory, Shared Memory & X Server Memory. What are the differences between them, and which reflects the actual RAM usage of the process?

Sorry if it's a newbie question :)

• Virtual memory accounts for all the libraries and executable objects that are mapped into your process, as well as your stack space.

• Resident memory is the amount of memory that is actually in RAM.

• Writable memory is the amount of address space that your process has allocated with write privileges.

• Shared memory is memory that may be simultaneously accessed by multiple programs with an intent to provide communication among them or avoid redundant copies.

• X server memory is the amount used by the X server for the GUI-session.

References:

• "Memory Management" on The Linux Documentation Project website

• Stackoverflow post

• "Shared memory" Wikipedia page

To add on to Ringtail's answer, the plain "Memory" column is Resident - Shared, giving you the memory that the process solely owns.