In Nokia cellphones (and probably in many other GSM phones as well), an operator logo is, as name implies, a logo that appears on the screen of the phone when the phone isn't used, and is usually by default used for the network operator's logo.
Naturally, operator's ego is not the only thing that can creep on the screen. At least in Finland, the cellphone operators offer a wide variety of logos and ringtunes. Many other companies also offer these services, probably some even specialize on them... The logos are ordered with a SMS message (in Finland the price is around 5 FIM).
These were sort of a mad craze at one time. Their popularity is understandable - everyone wants a cellphone that looks and sounds different from others - but otherwise the business is sort of strange. It's hard to understand that some people make millions with small 2-color pictures...
(I'll also be surprised if I find a teen homepage that doesn't have links to ringtunez and logoz. =)
A friend of mine - tigert - made an operator logo for me specifically. Available from Saunalahti, under titles "susi" and "susi2" =)