What is the meta? 1 2 3 4 5 Roles?

1-5 indicates farm priority, where the lower number gets farm if he's sharing a lane with a higher number.

1 is safelane farmer - your hard carry

He typically needs tons of gold, and scales extremely well with items. Farm farm farm, all the time. This is your hard carry that is going to pull your team through the lategame. This is the Lifestealer, Alchemist, Anti-Mage, Spectre, basically any hero that is going to be meh early-mid game, and a god in the lategame once he gets enough farm.

2 is solo mid

Ideally, they're mobile (to control runes), have AOE damage (to push lane and control runes), and have some other positive aspect... Some might be able to transition to a carry role if the situation requires it, others might be excellent initiators, others might be strong gankers, etc. The idea is they're going to get levels and farm fastest on your team early on and be able to leverage that advantage.

3 is offlaner

Offlane often has a rough time getting reliable gold or experience without dying, so you're looking for heroes who can stay in experience range relatively safely, disrupt safelane stacking and pulling by the enemy team, creep skip, and generally be a pain in the ass. Reading the patch notes, offlane will probably change the most in 6.79. Not sure how it's going to go.

4 is farming support/jungler

This is your 'semi support' or 'soft support', the guy who can really help your teammates get kills throughout the game with stuns, slows, and other utilities, but doesn't have very good, or very reliable damage and normally have a really really really good, teamfight altering ult which they need level 6 for in order to be relevant. These are yourObserver Ward, Dust, Smoke, Mekansm carrying heroes, like Sand King, Tidehunter,Rubick , Chen, Naga Siren.

5 is support

This is your hard support, the team's ward bitch, the guy whose always the reason your team loses. The Lich, Crystal Maiden, and a few other heroes. These guys can carry the urn and sometimes medallion or drums, but basically their only use in the whole game is to use one or two of their abilities to secure easy early or midgame kills. Eventually they really fall off into the game, but they pick up farm once the other players slow theirs down in order to be relevant. Often times they don't have an ult as enormous as the 4 role.

• 1 (safelane carry) might include Alchemist, Lifestealer, Gyrocopter, Anti-Mage, Spectre, Luna, Riki, Phantom Assassin, Chaos Knight
• 2 (solo mid) might include Queen of Pain, Puck,Outworld Devourer, Dragon Knight, Templar Assassin, Storm Spirit, Zeus, Shadow Fiend, Pudge
• 3 (Offlane) might include Nature's Prophet, Bounty Hunter, Lone Druid, Clockwerk, Dark Seer, Lich, Clinkz, Mirana, Windranger, Batrider
• 4 (Farming support/jungler) might include Enigma, Enchantress, Chen, Io, Vengeful Spirit, Visage
• 5 (Ward bitch) might include Crystal Maiden, Disruptor, Shadow Demon, Lion, Keeper of the Light

Two main questions at the start of a game...

Where do the supports go?

2-1-2 (one support top, one bottom) is common in pub games, but rarely seen in pro games. (may change with 6.79 patch). In pro games, we've mostly seen either defensive trilanes (both supports in the safelane, attempting to kill or zone out the enemy offlaner, stacking and pulling to maintain creep equilibrium and deny the offlaner experience, while ensuring safe farm for the carry) or offensive trilanes (both supports go to the offlane to try and deny the enemy hard carry safe farm... Ideally, to kill him and any supports in that lane.)

Can the offlaner successfully get experience/gold?

If not, the offlaner may abandon the offlane and farm up in the jungle. This is why offlaners that can jungle are often preferred in pro games.

Interaction between roles is too varied to really sum up, because teams play differently, and heroes in the same role play differently.

Alliance typically runs defensive trilanes with farming offlaners ( Nature's Prophet,Lone Druid), so they expect Bulldog to secure farm either in lane or jungle, and they don't expect much help from him early on. They're happy to sacrifice their offlane towers as long as they can keep farming safely.

NaVi tends to run more utility style offlaners like Windranger, Mirana, and Bounty Hunter. I think they expect Funnik to get involved in other lanes or teamfights quite early in the game.

Ditto for mids -- running Pudge mid, you'll expect ganks once he hits level 7. Running Outworld Devourermid, you may be much more content to let him continue farming, gaining levels and gold.

As for items, it depends a lot on what kind of hero you're talking about. You wouldn't expect a Queen of Pain mid to get the same items as a Dragon Knight mid. In general, your 1 role is going to build damage and survivability, and your other roles will be dictated by situation... Generally somebody else will build Mekansm (3 or 4), and somebody will build Blink Dagger for initiations (2, 3, or 4, or several). Disables are highly valued, so an INT mid may go for Scythe of Vyse. Melee carries may build Skull Basher/Abyssal Blade for more disables.