There isn't roles such as initiator, tank, nuker, etc. Its just things certain heroes can do.
The roles are
1. Carry (Faceless Void, Naga Siren, Phantom Assassin)
2. Mid (Puck, Death Prophet, Tinker)
3. Offlane (Tidehunter, Clockwerk, Bounty Hunter)
4. Support (Earthshaker, Bane, Shadow Shaman)
5. Hard Support ( Crystal Maiden, Lion, Vengeful Spirit)
Something you have to keep in mid which is different from LoL is that a lot of heroes can play multiple roles in dota.
Position 4 isn't always a support as well you'll sometimes have a jungler which can either be a support jungler or a more aggressive carry jungler depending on the hero.
Every support can pretty much play position 4 and 5. The difference is that the 5th positon usually buys the majority of the wards, courier, smokes, dust, etc and lets the position 4 try and buy some items.
Aggressive junglers include heroes like Ursa, Lycan, Nature's Prophet.
The more support junglers are heroes like Enigma, Enchantress, Chen