The built-in Voice Memo app can be handy for recording lengthy thoughts or instructions. After finishing your Voice Memo you can easily share it with someone else as an iMessage or SMS. Once they receive it they'll be able to play it directly through the Messages app.
If the recipient isn't using an iPhone, iPod touch, or iPad, but another smartphone like Android, BlackBerry, Windows Phone, or even a feature phone, they won't be able to receive an iMessage but might get the file anyway as an MMS message. If they do, whether or not they can play the audio will depend on what kind of file formats their device supports. (Voice Memo files are in .m4a -- or mp4 audio format.)
If you're having issues getting iMessage to activate, get more help at the iMore forums, and for more tips, check out: