The mobile phone can alert you of events such as an incoming call or an incoming message in a number of ways. The two most popular ones are vibration and ringing.
While vibration is pretty much self-explanatory, the ringing alerts can be of several types depending on the mobile phone.
In the past mobile phones used to allow only monophonic tones to be set as ringing alerts. With the advancement of technology, polyphonic ringtones also became supported.
Later on as mobile phones further evolved, they started using MP3 tones for various alert purposes. As more (presumably superior) audio formats became available (such as AAC), they were also added to the mobile phones' alert system.
Recently, mobile phones have even started to use short video clips as call alerts (not to be confused with video calls).
No matter what kind of ringing tones the mobile phone uses, users have always enjoyed customizing their ringtones by expanding the preset ones supplied by the manufacturer.