The main questions are:
1 How does a wallet or purse Bitcoin?
Bitcoin client automatically generate a portfolio that contains pairs of public addresses and their corresponding private keys. Public addresses are those that are - which can give to know to receive payments. The private keys, on the other hand, are only in your wallet (in the file wallet.dat).
Imagine that your public addresses are inviolable mailboxes that everyone can see, and in which everyone can park bitcoins, but that only you can open your private keys. Every public address is "opened" with a private key specific and impossible to reproduce. If you receive 1 bitcoin which was sent to one of your public addresses, the only way to eventually transfer the possession of the bitcoin (of "send it" to another person) is using the private key that corresponds to this public address.
While you keep the portfolio, you keep private keys that allow you to dispose of the bitcoin controlling that portfolio. That is why it is advisable to keep backups of the file wallet.dat.
2 - why it never will be more than 21 million of bitcoins?
21.000.000 limit imposed by the Protocol is arbitrary; What matters is that, according to rules that implicitly accept all those who use the system, this limit may not be exceeded, nor can alter the rate at which the money supply is increased. In this sense, Bitcoin is absolutely predictable - which is essential for a monetary system.
3 when will they occur the bitcoins?
The bitcoin are generated as a reward to the work of the miners, and that reward decreases by half every 4 years. Toward the year 2030 it will have generated almost all bitcoins that will exist, but the truth is that they will continue generating (increasingly less) bitcoins. In the long run only they will enter the economy small fractions of a bitcoin (IPDS) in the course of several years, but the curve representing the increase in the money supply will continue approaching asymptotically 21.000.000.