Bitcoin is a digital currency and you can pay with it, change it to other currencies, invest, and so on. What can be done depends on someone accepting the payment with Bitcoin, the change and in general behaves just like other currencies.
However, it has an important peculiarity: it is a cryptocurrency. That is, it does not depend on a central emitting entity, but on the network that allows it, which is a decentralized peer-to-peer network.
What does the above mean?
It means that it is a network of computers in which all are at the same level. I mean, there's no one above. These computers are put by anyone who wants to join the project in a free way and in return, the network gives them some bitcoins it create for this purpose. That is, whoever that wants to put high power equipment to make the book-entry (as do the servers of a bank) and the network pays for it. This can be done because there are systems in which it is mathematically impossible to cheat. If someone wants to do it, the rest of the network detects it and kick it out. There are no bills that can be copied here, it all depends on encrypted codes and systems continuously checking if numbers are ok. In addition, it would not be enough to deceive one, it would have to do it to all the net at the same time.
Also, since there is no authority above it is impossible for these privileges to be used to alter the behavior of the currency and thus to favor their own interests.
How do I buy and operate with Bitcoins?
To get Bitcoins, there are several ways, but the simplest way is to go to a Bitcoins exchange house. For example, on this page we can see many. It will appear after choosing the area where we say where we have the bank account and the method of payment. As a general rule, we will create an account at the chosen exchange and start operating. The process is exactly the same as with other currencies. In fact, depending on the house, you will buy more expensive or cheaper Bitcoin as it happens when we change from Euros to another currency.
To convert our Bitcoins into Euros again, we can do it at the exchange house. Just give the order and the bank income will be made. For both the purchase and the sale, it is normal for us to be charged a commission. This varies from one house to another, just as it happens when we change any other type of currency. So it is advisable to spend a couple of minutes comparing them to choose the Exchange house with the best conditions.
In short: Bitcoin has been designed so that no one can alter it. It has no central bank, peers can not cheat, the network has no owners and can not be forced changes because everyone must accept them. In addition, there is a limit in the money supply and nobody can emit currency with the consequent inflation.