Top is usually for tanks, but there are many champions that can go top lane as well, such as Heimerdinger, Vladimir, Riven, Jax, etc.
Middle lane: This person will usually go AP, but there are also AD mid laners such as Zed.
Jungler: Can be a tank or a bruiser (high attack but low/mid defence).
ADC: Stands for attack damage carry. Logically, they build AD. Their sole purpose is to farm and win lane so that they can become relevant later in the game.
Support: Sole purpose is to babysit the ADC and also have utility in teamfights. These are generally tanks because they build items that, while also giving them utility abilities, have defence stats (Locket of Iron Solari, Frozen Heart, Randuins Omen, Righteous Glory).
Terms
AD: Attack Damage, which is damage based off auto attacks.
AP: Ability power or Magic damage, which is damage based off abilties.
So you usually want a balanced team that attacks on several angles. If your team is literally all AD, then your opponents can counter you very easily by buying thornmails and Randuins, If you do not have one or two tanks, then your teamfighting ability will be severely affected. Here are some healthy team comps:
Top, AP (Examples: Rumble and Maokai, the latter having the bonus of also being a tank)
Mid, AP (Cassiopeia, Ahri, Leblanc, Azir, Annie, Viktor, etc)
Jungle, fighter (Vi, Lee sin, Kha Zix)
Bottom Support building tank, can be either AP (Morgana, for example) or AD.
ADC, always AD.
or
Same as above except top builds ADC. Sure they can build Banshees or Locket to counter your midlaner, but they also have the AD damage coming in. Preferable that the mid snowballs so that they have to counter it.
or
Top AD fighter (Irelia, Jax, Hecarim)
Mid AP
ADC
Support (AP or AD)
Tank Jungler (Shyvana, Sejuani, Hecarim).
If your support builds tank, then you have a nice 2 tanks!
As for Ahri, she is not very good in other positions. You can try her as support or in top lane, but your opponents can always take advantage of that. I suggest you try the jungle, because it is awesome:
- You will have no one harassing you will you farm (exceptions apply)
- It is always different
- You can build a variety of champions (Tank or figher, sometimes you can even go AP with champions like Diana)
- You get to play a role in all lanes.
- Can carry games too.
- You dont have to worry about last hitting.
Normally you can have crappy team comps at low levels because most times someone will just get extremely fed on either side. But you have to play good champs in ranked and later levels.