-san: Equal to Mr., Ms., Mrs., etc. in English. It is used when politeness is needed. For me, I use this for users I respect and/or am not familiar with, and users that I feel I would be disrespecting if I titled them as a friend ("kun" or "chan") unless they authorized that I may call them by a familiar honourific.
-sama: Used for very high respect. I usually use this for chat moderators that I have not befriended. If I use this for a user that is not a moderator, it means I respect them greatly and practically look up to them as a user. It also always means I have not yet befriended them.
-dono: Even greater respect than "-sama". It is usually translated to "lord" or equals to such titles. I use this for administrators.
-kun: A name for a male that expresses familiarity, and friendship. I use this for male users I am close with.
-chan: A name used for a female that experesses endearment. It is also used by other females to show familiarity and friendship, or between two lovers. I would use this term for female users I am close with and/or "love interests".
-sensei: Used for teachers, doctors, masters, or any other type of authority figure. Can be used in complete replacement of one's name or put at the end of their name. I use this title for users that have taught me something or are teaching me something.