Twitter visibility

1. When we follow an account, we see followee's own updates not others that s/he follows, unless we already follow ones of them mutually.
2. People will only see replies in their home time line if they are following both the sender and recipient of the update. People with protected accounts cannot send replies to people who aren't following them, and mentions won't be seen by non-followers either. [from: "What are @replies and mentions?"]

Lists
As simply stated in Lists' privacy setting:

• Public — Anyone can subscribe to this list.
• Private — Only you can access this list.

We see lists when they are set public, and cannot see/access private lists (even we specify exact URL).  This rule is not matter the lists' owner is normal(public) or protected account.

What connection can we make with other people? Given that they are: »	Normal/public account	Protected account
Follow them	Yes	To request
Keep them in list	Yes	Available when we've already followed
Follow (subscribe) their lists	Public lists only	Public lists only (needn't request)
See their private lists	No	No

Whom do we see inside a list? Given that they (not list owner) are: »	Normal/public account	Protected account
List is public	Visible	Visible if we've also followed them
List is private	List is inaccessible	List is inaccessible

Also notice that we may put people (of normal/public account) into our list(s) without following them: i.e., they are not shown up in our main-stream time line.


Further reading

• Twitter blog: "Soon to Launch: Lists" (Sep.30'09). "There's a List for That" (Oct.30'09).
• Josh Catone "HOW TO: Use Twitter Lists"; Mashable (Nov.02'09)
• Mashable: "Twitter Lists, Resources & How-Tos"