Not a stupid question at all.
Many 'energy saving' lights have a standard BC (bayonet cap) or ES (Edisson Screw) fitting and are a 'drop in' replacement for regular lamps.
On the other hand, some fluorescent lamps have a completely different fitting (bi-pin, 4 pin, etc.) and so need a specific fitting to take the lamp.

Also, many light fittings, especially with lampshades are rated to a maximum wattage of lamp. A typical table lamp with a 9" paper shade may be rated at 60W max. If you use a 100W tungsten lamp, you are in danger of scorching or setting fire to the shade. Many newer designs of lamp or fitting will specify 'energy saving' lamps as they rely on a much lower heat output from the lamp.
There are also standard 'pearl' lamps that look to all intents and purposes like a regular bulb, but may have an odd-ball fitting to prevent insertion of a regular bulb. Especially where lighting circuits these days use far lower currents than traditional lights.
A normal household lighting ring is rated at 6A and will use 1mm cable. That is 1.44kW of lighting using traditional bulbs. Some newer establishments have specific energy efficient fittings and much smaller cable, which would be overloaded by standard tungsten lamps, so have a specific fitting to prevent incorrect lamps being used.
Example

One place that springs immediately to mind is in Farnborough, Hants, where a work colleague has just bought a flat in the new 'Concept' development. All the light fittings look like regular ones, until you look closely and see that the fittings and bulbs are different. What looks like a standard light fitting in fact takes the bulb base shown above. The down side is that these bulbs aren't often 'off the shelf' at B&Q!!

And contrary to popular belief, they are not 'fit and forget' for eight or ten years....40% of energy saving bulbs suffer from electronic failure in early life that means they need to be replaced much earlier than the manufacturers would have us think.
A bulb may say 80% less energy and a 8 year life....I have never seen one 'guaranteed' for 8 years!!!
So yes, you CAN get 'low energy fittings', but low energy in the sense that they only fit low energy bulbs!!