I would like to know the difference between MP3 series mobiles and Walkman series(Sony Ericsson) mobiles, what type of datas are stored in each and how are they working.
:)Najeeba.

There probably would be no difference at all.
An mp3 is just a type of audio file, it more than likely would be able to be played on any unit.
If you wanted to use an audio file as a ringtone though, you may struggle.
Some mobiles allow you to use MP3's for ringtones, others use WAV files.
there are some free programs out there where you can convert one file type to the other. There are even some websites which will do it.

Also, 'Walkman' is just a trademark of Sony used since the 80's that they used on anything that was a personal music player.