Using the #tag in Twitter:
In Twitter, the "hashtag" is used as a way of grouping or categorizing tweets that eventually become indexed for an easy find when searching. For instance, during the US presidential debates or Mumbai bombings or Iranian elections, tweeple used certain hashtags in their tweets when discussing the debates, updates on the victims of the Mumbai bombings and election results. This way, people could easily search in twitter specific news about a certain event and then follow all comments that include the hash tag. So in real time, you can follow the comments and respond to them, creating a conversation about a given topic. Thus, if an item or word has been hash tagged by many people, such as #follow friday, then it becomes easy to find.

Others use the # tagto denote discussion of events, groups, TV shows, movies, themed discussions, world events, and more.

Using the @ sign in Twitter:

1. Use it to capture the attention of someone in a group of twiple in twiland.
2. It is a sign of attribution when you retweet a message by another user (eg. RT @subinkuta). 
3. Use it to suggest twiple to follow others or something, eg. #FollowFriday @subinukta
4. Using the @username (eg. @subinukta) the message appears in that username's twitter inbox.
   NOTE HERE:
   This should not be confused with DM (direct message) i.e. sending a direct message to twiple. Anything sent via DM is out of public and can only be seen between the two people who send direct messages to each others' inbox. Also, you can send an @ message to anybody – whether or not they follow you BUT you can only send direct messages to people who follow you.
5. When you @ somebody, that message is posted in public, although directed to someone whose name  appears after the @ sign.  All the twiple following you and the person you @, will be able to see your message.  For example, let say I follow @jkmutta and you don't, then each message I @jkmutta, you will not be able to see but all the twiple following me and jkmutta will be able to read it. HOWEVER, if you begin writting a message by anything even a - or ? or . or any symbol then followed by the @ sign and username, e.g, '-@subinukta, can we meet today?' then everybody will see that message where or not they follow you two twiple. So, if you  follow Oprah Winfrey or Elen DeGeneres or Robert Kiyosaki or Anderson Cooper,  don’t assume they will see your messages just because you used the @ sign, you may want to try to catch their attention by DM them if they have that feature allowed (good luck). BUT if you want to send a message to somebody you don't follow and you can't DM them, then try the @username.
   
6. If you want to spread a word, better use the #tag instead of the @ sign because it limits the number of people who can see that message. For example: if you tweet, '@subinukta has as a good post about scholarship tips at wavuti.com' that message will only be seen by people who follow you and subinukta, you would then do a good favor if you just said, 'subinukta has a good post about scholarship tips #wavuti.com' without using the @sign.
   

Did you know:
That anybody can read your tweets if they click on your username? So, if you think using the @sign will prevent some people from seeing your tweets, the answer is NO, it won't,  people can still see them when they click to read your profile. If you want to say something in private, then DM them or send an email or better still, call them by phone.

PS:
Tweeple or Twiple are People who have twitter accounts and tweet periodically.
Twiland or Tweeland is a virtual world where twiple or tweeple live and tweet.
If you have a twitter account and you do tweet, then you are automatically a twiland resident.