Hello, I'm new on this community of cryptocurrency and I have some doubts about the market nowadays
None of those questions are really related to the market but rather on mining.
1) What the "Best share" on the cgminer windows means? What's a share to be more exactly and how does it works, what's the difference on share 56 and share 3.8k?
The solution of a block is called 'nonce'. When you mine in a pool you mine 'fake' pool blocks having lower difficulty than network. Those 'fake' blocks have 'nonces' like the network block but they're called 'shares'.
Why is this the case?
When the pool makes you mine at diff 2, every solution you send will have 'value' 2. At diff 4, every solution will have value 4 and so on.
However, the real 'value' of a magic number you send is not truly related to the difficulty the pool requested: it is a function of the number you found.
So, the pool might request you to mine at diff 16, you find a solution to be sent back, it might have difficulty 16.123 or 143k.
All the shares better than requested difficulty are accounted (at pool diff) but if a share also have a higher value than a real block then you found a network solution (found a block).
2) Is it better to mine on a pool with thousands of users or not? The profits are the same if you mine on a pool of 1.9k online users and a pool with only 5 users?
You're taking it a bit on the extreme: it is hard to believe the two pools can have comparable hashrate.
The number you have to look for is the "average round". Pools with more or less the same average round time will give more or less the same payout. Caveats apply.
3) How the "blocks" works? What's the lucky percentage? These blocks can be found by anyone or only with those who have the best KH/s rate?
The percentage is a function of difficulty, the whole idea of difficulty is it counts how many attempts you must try before you find a block solution.
Note: difficulty is a pooled mining concept and convenient for human beings but it does not exist at network level. Network level works on 'targetbits' instead.
The properties of the hash functions involved guarantee the solution(s) are randomly scattered. Everyone can therefore find a solution but of course an user twice as powerful as you have twice the chance to find one in the allotted time as he throws twice the amount of dices.
4) I have only one R9 290x, is it worth to mine LTC on a vardiff pool or is it better to mine some other altcoin with low difficulty?
LTC hashes Scrypt. Scrypt is ASIC game now. You have no chance to compete with GPUs. They've been out of the game by a year easily.
Note: difficulty for alts is not comparable. For the purpose of explanation let's say some alts measure diff in inches and some other in yards.
How difficulty works? Why last month the difficulty was x and this month is y? Does the difficulty only increase or it decrease too? Why does it increase and decrease and is it possible to predict it?
The network wants a block every X seconds. Every once in a while, it will count the amount of blocks and figure out they have been more frequent than predicted. If so, the network decides to rise the targetbits requirement. Difficulty therefore goes UP.
Maybe the network expected 120 blocks in a time frame and it got 100 instead. Then difficulty will go down.
It is therefore possible to predict it by running the computations in advance. Due to decentralization there are some extra complications predicting it accurately is much harder. Predicting it over long time frames is pretty much impossible.
