After the birth of the MP3 file format, there remained a problem, which was inclusion of textual information i.e. some information about the audio file.At that time a person named Eric Kemp alias NamkraD suggested a fixed-size 128 byte tag inside a MP3 file which would include would include title, artist, album, year, genre and a comment field. And this person himself implemented his idea and people welcomed it with open arms. Not much later Michael Mutschler, extended this tag which is called ID3 tag. He named his version ID3 v1.1. The current version of ID3 is v2.2.
You can read more about ID3 here http://www.id3.org/intro.html

Most of the CD ripping softwares include a custom or a default ID3 tag, but just in case your MP3 file does have an ID3 tag there are a plethora of software which will do it for you. The most famous, simple and easily available being Winamp, which has an in-built ID3 editor. In the Winamp playlist window just right-click on the playlist item whose ID3 tag you want edit, and select File Info, and you'll get a dialog box where you can input all the info about the song.
Yes I use that but thought that they keep it in seperate file but now I knew that they use the same file to store the data about the file.