(You can view this article in Hungarian on the Rejtett Uradalom site.)
The D&D design team intentionally reduced the number of energy classifications so we didn't have the problem that 1E/2E had where fire and magical fire were two different things, leading to idiocy of spells that created magical fire and spells that did not. Now it's just fire, whether caused by magic or not.
There are only five types of energy damage in D&D: acid, cold, electricity, fire, and sonic.
Acid includes liquid acid, things like chlorine gas (green dragon breath), and base/alkaline substances (such as lye) that chemically speaking are the exact opposite of acids but do similar damage to tissue. In short, stuff that damages your body by direct chemical action (rather than the side products of those chemical reactions, like alchemist's fire producing fire that burns you).
Cold includes environmental cold, supernatural cold, sticking your tongue on an icicle, etc. In short, anything that harms you because it is significantly colder than what your body is used to.
Electricity includes natural lightning, electric eels, car batteries, magical lightning, etc. In short, anything relating to the movement of electrons or the potential to do so, whether expressed in amperes or volts.
Fire includes environmental heat, friction burns (ones that don't abrade the skin from friction, just build up heat, like rubbing your hand on someone's arm for a long time, not sliding down a rope with your bare hands), normal fire, magical fire, other sources of heat that don't include actual fire (boling water, lava, steam), etc. In short, anything that harms you because it's significantly warmer than what your body is used to.
Sonic is loud noises (audible to humans or not) and intense vibrational energy (whether communicated directly, as from a screaming weapon, or indirectly, such as feeling your guts churn from being too close to a bass speaker at a concert). In short, energy of motion/vibration that isn't a result of "linear" kinetic energy from momentum (such as a swinging sword hitting you). [This is one of those times where using physics to describe something in D&D falls short, since vibrational energy--sound--is technically a form of kinetic energy, but oh well.]
The various endure/resist/protection from energy spells completely protect you from the actual damage caused by these sources. So a ring of major fire resistance protects against fire, steam, lava, and boiling water.
Secondary effects aren't necessarily prevented by these protective measures unless they directly result from the primary damage. For example, while your ring might protect you from the boiling water's heat, you can still drown in it. But an electricity spell that stuns with a jolt of electricity you would probably be negated by a ring of minor electricity resistance as long as it prevented all of the electrical damage A spell that immobilized you in ice (but did no damage) wouldn't be stopped by a protection from cold spell, but a breath weapon that did damage and gave a -2 penalty to all checks from acidic fumes probably would be entirely ignored by someone with protection from acid.