Nokia's 'essential' patents are tied into the standards, therefore they are available to anyone under "FRAND" (fair, reasonable and non-discriminatory) licenses.
Apple were in negotiations with Nokia for the 'essential' standards related patents and the negotiations broke down.
This is how it all happens:
- Nokia want "more" from Apple since Apple: 1- have acquired a significant market share (at Nokia expense) and 2- have a number of desirable patents for Nokia to license. Consequently Nokia want a higher license fee than 'Cheapo Cell Phones Inc' pays and/or a cross license to Apple patents.
- Apple want just the license, at 'Cheapo Cell Phones Inc' rates (because, lets face it, the iPhone is nothing but an iPod Touch with a cheapo cell phone lashed on the back).
They cannot agree, so they go to court.
Nokia sue the only way they can, for infringement of their patents. Apple counter-sue with everything: invalidity of the Nokia patents, non-FRANDy type behaviour and for infringement of Apple patents.
In the end, there will be a decision in which Apple will pay Nokia a license, and possibly will license some of their own patents to Nokia.
Bottom line, its a negotiation fail, lawyers get rich, price of Nokia phones and iPhones goes up to pay their bills.