Using emoji keyboard on iMac, iPhone, iPad, iPod

For iMac ( Yosemite OS )
Press Control-Command-space bar and an emoji picker will be displayed.

To Use emoji (emoticons) on your iPhone, iPad, and iPod touch:
Turn on the emoji keyboard
Tap Settings > General > Keyboard.
Tap Keyboards.
Tap Add New Keyboard.
Tap Emoji.


You can use emoji in Messages, Mail, Facebook, Twitter and more.
To open the emoji keyboard, tap Smiley face icon, or press and hold Globe key and select Emoji.

More about emoji characters read here: