@Tijnn.:
How does the hero system work?
Like gameplay wise?
There's weekly heroes that are free to use every week that you can choose from. The heroes can be divided into basic categories by the players into things such as Tank, DPS (Damage per second), Pusher (Can clear up lanes real fast to help take out opposing towers), Nukes (I may be wrong and it might just be dps again, but I think characters that do a lot of dps in an AoE is considered a nuke), Support, and a few other categories. A hero can fit into multiple categories and may be in one category in one game and a different one another game depending on which rune build you make and what items you buy in the game.
You start each game with 435 gold I think, and you gain gold over time once minions have spawned and get gold and exp for killing minions. Each person on your team gets 130 gold and exp for towers destroyed, and you get 400 gold for a champion killed. So you buy items that increase different things, such as Attack speed, attack power, ability power, defense, armor, health regen, lifesteal, etc.
Items may also have unique passive powers such as Mejai's soulstealer which you get 1 stack for assist of killing a champion and 2 stacks for killing a champion yourself. Each stack gives you +8 ability power and it maxes at 20 stacks, and once you hit 20 stacks all of your cooldown times are reduced by 20%, upon each death you lose 1/3 of your stacks. Making it a favorite among mages who have early game burst damage to get early kills. Another passive ability is Guardian armor which upon death instantly revives you with a set amount of health and has a cooldown which I can't remember how long it is. Which makes it great for tanks. Or maldred's bloodrazor whose unique passive is that on each hit it does extra damage equal to 4% of the opponent's hp.
To use unique active abilities you have to hotkey an item slot in the option menu on your keyboard. An example of a unique active ability would be Zhonya's ring, which upon activation places your character into a stasis field for 2 seconds stopping all damage. Or Deathfire's grasp whose ability does damage equal to 30% of the target's hp.
You gain experience by laning (staying close enough to enemy minions when they die.), killing champions, destroying towers, or jungling (killing minions in the jungle, which only certain characters should start the game jungling.) The benefit of jungling is that in a 5v5 you normally have 2 top 1 mid 2 bot laning. If you have a character that can jungle though you can have only one person in either the top and the bottom lane and when someone is in a lane by themselves, they don't split the exp with someone else. They do have to survive in a 2v1 situation, but staying behind minions or tower hugging is easy enough.
Each hero has 4 abilities, you start the game with 1 point to put into one of your first three abilities, then you level up with enough experience and get an ability point per level. Once you hit level 6 you can unlock your fourth ability called your ultimate. Ultimates are generally extremely powerful abilities that have long cooldowns. Most ultimates are for burst dps, though there's also support. They're usually reserved for killing champions. I know there's a lot more to it and my mind's drawing a blank, but it's a lot funner to learn by playing.