From a practical point of view, this means that drivers of non Spanish registered cards who are caught speeding or using reserved lanes in Spain can expect a fine through the post to the registered address of the vehicle.
My understanding is that at the moment, if you're stopped for speeding by the Spanish police you'll get an on the spot fine if you're in a UK registered car. If you don't have the cash to pay the fine you'll be escorted to an ATM and if you still can't get the cash then your car will be impounded. If you're driving a Spanish registered car you'll get a ticket sent to the address where the car is registered.
If you get caught by a gatso speed camera in Spain then the UK government currently has no legal obligation to provide the details of the owner of the vehicle and you could argue that the fine is not enforceable - unless you return to Spain in the same vehicle of course.
From May we need to have passed a UK law that mandates the sharing of vehicle ownership details with other EU countries, and presumably makes the collection of those fines enforceable.
In theory, once this system is in place the Spanish police could drop the on the spot fines for EU registered vehicles, but I doubt that will happen.
This link has more info on the EU directive behind the stories that are in the press at the moment:
http://etsc.eu/faq-eu-cross-border-enforcement-directive/What isn't mentioned in the directive or any of the news articles I've seen is what happens regarding points on your licence as a result of the offences covered by the directive.
Pete.