difference between item.update and item.systemupdate

Item.Update()

Updates the item in the database.
Updates the “Modified” and “Modified by” values.
Creates a new version

Item.Systemupdate()

Updates the item in the database.
No changes in the “Modified” and “Modified By” fields.
No new version.
Triggers the item events.

Reference:

http://www.c-sharpcorner.com/Interviews/answer/6620/what-is-the-difference-between-item-update-and-item-system