As its name suggests, an "emoticon" is an image or icon usually used in e-mails or other Internet communications and meant to convey an emotion from the sender to the receiver. For example, the heart emoticon -- which you form using the character sequence "<3" (without the quotes) -- is meant to show love, care or concern. How you respond to receiving the heart emoticon depends on your belief in -- or assessment of -- the sender's intentions.
Respond using a "kissing" emoticon if the heart was romantic in nature and you want to keep escalating the conversation. Form the kissing emoticon by typing the characters ":-" (minus the quotes) -- or in programs such as Gmail Chat, ":" (without quotes).
Use another heart emoticon if you want to echo an identical emotion to the one your friend sent. Copy the heart and paste it into your reply or simply type "<3."
Make a standard smiley face to signify your happiness at receiving the heart. Type the characters ":-)" -- or in some chat programs, simply ":)" -- without quotes.
Type a text-based message if you don't care to use another emoticon. Explain how the heart emoticon made you feel, whether it was loved, touched -- or, if you didn't want or expect to receive it -- perturbed or upset.