A few questions about warding

I'm trying to learn how to be a good support (although it's kinda frustrating in pub games, you really don't get any recognition) and I've read the ultimate guide to warding, but there are still a few things unclear for me.

First, the guide is all about "put a ward there", but which ward? There are two of them, the observer and the sentry, and as far as I understand the difference is that the observer last longer while the sentry has true sight, but I'm not sure about other things, for exemple: are both wards invisible to the enemies? They have the same range?
And if the guide was about using the observer wards, when it is useful to use the sentry?
If you could explain these things to me I would really be grateful!

Place observers, they have a bigger range and last longer and both of them are invisible to enemies You only gain sight of wards with true sight
Use sentries when the enemies have some kind of invis heroes since true sight gives vision of them or you want to de-ward the enemies' wards.

The difference are:
Observer Ward cost 150 gold for 2 wards, have a wide sight range of 1600 at all times and lasts for 6 minutes.
While Sentry Ward costs 200 gold for 2 wards, have a sight range of only 70 at all times, but they reveal invisible units around them in a range of 800 and last for only 3 minutes.
They both come in a set of 2 wards, have 200 HP and are invisible.

So, you place Observer Wards to have a vision of an area, like near runes, or places where enemies might gank you from.
While you use Sentry Wards to reveal invisible heroes in an area where you think they may come into.

So, Observer Wards should be bought everytime, to have vision of runes and to be safe from ganks, while Sentry Wards are only bought if you are against invisible heroes, or if you want to destroy enemies' wards if you are sure about their place.

I have an upcoming guide for this, however it is taking ages, I have +100 ward spots and how to counter them and how to effectively use smoke.

Thanks for you answers! So if I have true sight and I can see the enemy wards, I can destroy them right? I guess this is what they call "de-warding"

I have to admit thought that in the low tiers pub games I'm in, I rarely see people using wards. But I want to learn to place them now so I'm used to it when hopefully I'll have to deal with more experienced teams.

Yep

Buying wards wins games and I am glad you recognized that ;) good luck with this knowledge.

it's hard to learn how to place sentries, when you're not dewarding.. if the other team are not warding don't deward (you're wasting money). Instead of sentries, learn how to utilize smokes.. much better for early learners - as smoking is a very high skilled item to use effectively.

when you know WHERE to place observer wards, that is when you will learn how to de-ward, as you will know where most obs will go - these come hand in hand.

smoke of deceit was a new item to Dota1 - and I never really learned to use it then, so few people even use it now - as a support, smoke of deceit is a god send!

Yup, it's all about experience, trial and error. I got Observers the other day as a level 3 Clockwerk against a mid Pudge so I can spot his hooks. He bought his own Sentries the exact same time. Funny how these things work...

Also take note of the common warding spots, because if you're in a mid-lower bracket its pretty safe to assume that players are placing their observers in obvious locations, making it easier to de-ward the other teams wards.
Another thing to note is the importance of warding jungle camps and ancients especially after the International. A lot of players love Navi, and there will probably be more than a few that try to emulate funn1k's ancients stacking, not to mention the large pool of players that understand how to stack, and those that know how to stack and clear them at lower levels (there are players that will clear stacks, or stack and clear ancients like Alchemist, Tinker, Lycan, Ursa, Doom, just to name a few that can or generally do it early).
Warding the safe camp is also a good idea generally if they're not defending it, because it prevents them from the easy lane pulls close to the safe lane T1 towers.
Angelobangelo is a good resource for support info, as well as Nubtrain and Wulfstan (I've played with Nubtrain and Angelobangelo a few times, and Wulfstan because he's another one of the higher skill bracket players I've seen while spectating Nubtrain and angelobangelo games).
Long winded but hopefully it helps a little, and if you get a chance, do what myself and a bunch of the players in the group I'm in have, and play with high skill players and be open to their constructive criticism in game it helps improve your decision making, or in this case warding/de-warding.

I was also considering something... WHEN you should start to put the wards? I guess the rune wards are useful from the beginning, to see where the rune spawn, but it would be a waste to put wards in the other positions at the start, right? Maybe when the heroes are around level 6 and there are more chances to get ganked?