Here's how copyright laws work in most parts of the world:
When you buy a CD, you don't own the music, you only purchased the *rights* to use the music in some ways. These rights are endowed in the owner of the CD.
If you rip the song and send it to a friend or even relative, you are enabling him/her to listen to the music he has not purchased any rights for. Since you are still the owner of the music, this would be illegal.
If you gave away your CD, you would transfer the rights, as the owner of the CD has now changed.
So, to answer the question:
Is it illegal to send one song from a CD you bought? Yes, it is.
There are legal uses for copying music though - as long as you're using it for personal use, i.e. for backups or use on an iPod, it's legal.