If you are using iOS 5.x or higher, emoticons (aka emojis) come stock on the device. You can activate them by going to the General section of your Settings app, then going to Keyboard > International Keyboards, then choosing "Add New Keyboard." Scroll until you see the Emoji keyboard and select it.
Once you do that, you will see a world icon on your virtual keyboard. You toggle between the keyboards using that world button.
Of course, key strokes also create emoticons on some web sites (and forum software, like this forum's). For example, a colon and the capital letter D will produce this emoticon at this forum:
There are more forum icons available (if you are posting from the web version).
One thing to keep in mind is that the emojis that are produced from the default keyboard are Apple-specific. Only oth Apple devices can appropriately display them (other programs/devices will only see squares or other weird symbols).
This is probably more info than you needed - hope some of it is helpful.
Marilyn