5 enemies in the same lane

Healing is not necessarily required. I mean, Lich and Crystal Maiden are lane supports, and they don't have heals (except for CM aura, but you don't really need to stay in the same lane do you?)

What is important for a lane support hero is that he/she has the following traits:
-Can harass
-Is item independent
-Has good skills in general early on

Maybe I phased it wrongly. lane-supports should have something that can be given to carries to assist them. Like Lich has Frost Armor and CM have Arcane Aura. Many opinions vary, though.

I would agree with that.

I do believe being ranged is pretty much a must, though. The idea of a lane support in general is keeping your carry alive all while letting him farm. Unless it's a trilane, where a melee support is valid.

:D
So lane-supports have buffs/heals right ?
Should they harass their enemy heroes ? I'm asking this because if you harass , there is a chance that they'll stick to their tower so your partner won't be able to gank .

Not necessarily buffs and heals (though those are definitely good) they could also have some offensive tools like spammable spells, like Arcane Bolt or Shadow Poison.

As a typical lane support you WANT the enemy team to go near their tower and stay there. This way, your carry can get some uninterrupted farm. And both of you can get EXP.

Another important thing as a lane support is to pull- if your carry is free farming, then most likely he will inevitably push the lane, even if he's just getting last hits. If the enemy team is staying at their tower and the lane creeps will not go to their side, then that's a HUGE advantage for you and a massive disadvantage for your opponents.

Never auto attack enemy creeps, only go for last hits if your carry can't get to it in time or you're fighting under your own tower. Deny as much as possible and attack the enemy carry when he comes in for last hits, attack the enemy support when he strays too close or comes in to attack your carry. Try not to move around the side of the creeps as then they'll start to chase you making it harder for your carry and putting you in danger. Nothing more embarrassing than death by creep.

If they stick to their tower, they get no exp or gold and you win the lane. Of course, if you don't see them, it probably means they're ganking, not just idling.

Why isn't venomancer a lane-support ? I mean when vengeful is one , he should be one too , shouldn't he ?
He has that DoT , his attacks and a ward . He can perfectly harass enemies !
(Can supports babysit too or should they necessarily be a lane-support ?)

Every support can babysit.Those that don't have any heal/buff ability can just buy consumables for the carry, that is also a type of babysitting.

How to know if a carry is a hard-carry ?
What's the difference between hard carrys and carrys ?