The first two lines of each letter will be shown in your mailbox
Other people will see the first two lines of the last letter from you in their mailbox
You can choose invisible mode
You are not seen in the list of online users
The chat is turned off in this mode
The date of your last visit is shown as the last one before you turn this mode on.
In the search result you are not moved forward (usually if a person logins into the site, he/she is moved forward in the search result and is shown on the first pages so that people see who is online now and can communicate with them more), so your profile will go more and more down in search results according to the last date of your visit of the site, that is shown in your profile.
High ranking in the search (if you won't choose invisible mode, of course)
Your profile in the search will be singled out by special color