Smileys are a stylized representation of a human face. Smileys are used to express your feelings. You can add smileys in twitter to interact with your friends. Here is a how to guide for adding smileys in twitter

You will be using a userscript in your Firefox to put smileys in Twitter. So you will be needing Firefox, if you dont have firefox you can download here. Now install the Greasemonkey. To get the smileys visit here and install the script. Restart firefox and you are done.

Now navigate to Twitter and Enjoy tweeting with emoticons =)