What's the XID?

http://www.postgresql.org/docs/8.1/interactive/maintenance.html#VACUUM-FOR-WRAPAROUND

XID is transcation ID?
so, "select * from mydb" is a transcation?
if i executed "select * from mydb" twice, the XID wil be increased by 2

or ,just the "update","delete"  command can be counted
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