It was a class/skill system where you would pick a class to start off on the server and you could level it up and as you would level it up you'd gain access to skills passive and manual use skills.
When you unlock these skills you'd use them to basically well do what they are intended to do for example there were passive skills like life steal when damaging someone or a mob you'd gain 10% of the damage back to your health, or a more non-combat passive which would maybe just be a Mining which all it did was depending on your Y distance down you'd randomly get a drop of whatever is found at that Y example like 1-18y something like that mining stone you'd have a 0.01 chance to randomly get a single diamond the chance would go up every level till you cap others would be like 18-like25 would be redstone 25-like35 was gold and higher was iron then coal etc.
The combat which Heroes gave was simple but you had to generally remember what your skills did to win most times example you'd be fighting a paladin who has ... Smite, Heal, bolt there would be more but those i remember right away... but you wouldn't know what those did unless you checked and then remembered even while in combat with them so Smite was just a contact damage ability that you'd be need to be within 1-3 block radius and you'd deal X amount of damage to them with X cooldown to it, Heal which sounds easy to understand could have been specific like Heal... Xhealth but maybe on when not in combat so you'd know to stick on to him when he was low or half so he could just run and try to heal... Then theres Bolt which you'd probably guess what it did but it was just a bolt of lightning that'd strike the ground or target dealing X damage with X cooldown.
There was tons of skills and even more now you can check the plugin on Spigot there are skill packs now sadly it costs money which is one problem why there are no servers anymore... but there were skills, modifiers, classes, professions, highly and i mean HIGHLY configurable to the point to were you could change the health of all classes to what ever you'd want even have 1,000 health if you'd like but you have to balance damage to match it so one class couldn't be OP but there was Mana so you could add mana cost or make that class not dependent on mana and just off cooldowns. Professions were just side classes that'd give you simple skills that mostly weren't used in combat how i explained the Mining skill there were more like Farming, Woodcutting, etc.
But yeah there was alot of things good and bad you just had to find the right server OR do what i did and help servers balance their classes which i saw alot more fun in doing but the PvP was so much fun also it's compatible with (i can't remember the plugin name) the plugin that'd you be able t make custom mobs with skills etc but it worked very well with dungeons etc.
Theres alot to explain so i won't be able to explain all of it the spigot forum will give alot more information.
www.spigotmc.org/resources/heroes.305/
