A preset is a texture or "skin" you put over your character. You make it using reasonably good art programs (Meaning, no Microsoft Paint but more like GIMP, or Photoshop, etc.) and then when its finished, you can do whats called "exporting" it which converts it into one file. That file will allow you to put it on your character in multiplayer. If you want others to see your character with the skin over it's normal character-creator markings, you must put the file that results from the export on a website (Upload it to something like Mediafire.com or something along those lines) then give the download link out to your friends.
I know that must be a little confusing but I hope I explained it well enough. ^^;
