Using “Smileys” or “Emoticons” in text messages is very common today. All phones today have a fair selection available to you to use in your messages. On Android phones too, you have a dedicated button on the stock android keyboard for inserting these emoticons. Even third party keyboards like Swype have this dedicated button which brings up a full list of emoticons.
Towards the aim of using these effectively and the right context, you may feel the need to know what these emoticons mean. There may be a few that you have yet not come across. To address this query, while you have a dedicated keyboard button for emoticons, you also have a menu option to see the meanings of each one of them.
You can click on the one that you deem appropriate for the message to insert this into the text area. This feature is available in the text messaging area and also in some third party apps. I have not seen it available in the mail app though, which is a little surprising.