Thanks for your extensive response. It answered my question. However a new quesiton arose. What is the maximum size of data which could be stored in the tangle with one single transaction? Since there are no fees I could theoretically store a mass of data without paying anything. If I got it right, it is just enough to confirm two other transaction to attach my transaction to the tangle.
I consider to buy IOTA, but I still have problems to understand it fully. I read about Carpass project and about Satoshipay what plans to switch from Bitcoin's blockchain to IOTA's tangle.
If I got it correctly, Carpass allows to observe cars by writing data into tangle, but how can data be stored? And if there is no need to pay fees, what value does IOTA tokens have? If I create a transactions with no fee, why do I need to own IOTA tokens then?
The data is stored in the Tangle which is the distributed ledger for IOTA. Basically you can do two things with IOTA, transferring data and transferring values(coins) with no transaction fees for both. When you do economic and business activities such as buying and selling resources (like data, storage, computing power) in the IOTA network which you can look as a marketplace, you need the iotacoins as the currency to settle the transactions (make the payment) although there is no transaction fees. So the more business activities, the higher demand of iotacoins, the higher value of iotacoins, based on the supply and demand relationship.