Thanks Amph and ikydesu.
I understand, so sign message is optional. In general, when people ask to sign a message? When you buy BTC? When you sell BTC? Or when do you buy a product with BTC?
There is no general case but people ask mostly when they suspect you are an impostor.
So every time that I do a transaction my wallet software generates a transaction id (tx id), right?
It's not your wallet software. Bitcoin transactions contains txid but is not generated by your wallet.
I didn't understand what you mean by "number is txid". 6e6828e9c03c998aa2b80a5c8c4d9fed69331c226074ae5747bc1971e1d60d94 is txid of that transaction.
This wallet 19ro4oQWS3RfVFtJRGRpaEBVuyPUoyzqi9 sent 0.0495 BTC, right?
Yes and that is address, not wallet.
It also display:
1NEA6ckomtrjVCbgXpYWQ1s6D38ECTwNF3 - (Spent) 0.0294 BTC
1B9Hbrcd9sSMg4acHBttjvYE9kucmNEG1k - (Spent) 0.02 BTC
Does it means that the guy that received the money already used 0.0296BTC?
Those two addresses may not belong to same persons and yes, those coins were already spent. You can see the transaction which uses those outputs by clicking 'Spent'.
What means the included blocks? Is it the main big blocked processed by miner pools with tons of transactions?
A block is a set of transactions. Your transaction needs to be included in block or else, that transaction will be dropped and the receiver won't get Bitcoins you send. You can see confirmations in transaction details. 1 confirmation means it was included in a block, 2 confirmation means a block was generated over the previous block and so on.
I see a huge number of confirmations. What happen if this name is small? What is a safe number?
Less confirmations means less blocks were generated over the block this transaction was included. 6 confirmations are generally accepted confirmation number though 2-3 confirmations are probably enough. However, it depends on the party involved. But now, 30 confirmations are recommended because of the blockchain split.
Good.