Hello Giovanni
There are various terms used to specify the speed at which data moves on a network. Speed, bandwidth, bit rate, throughput, data rate… All of these terms, and others as well, are used differently by different people and in different contexts. However, generally accepted definitions for the terms you used in the context of networking are:
Bandwidth - The maximum rate of data transfer across a given path. So, for communication over a FastEthernet link, for example, the available bandwidth is 100Mbps.
Bit-rate - The number of bits that are transmitted over a unit of time. This may refer to the actual bit rate at a particular instance, or the maximum available transmission rate over a particular path. This depends upon the context in which it is being used.
The context, as mentioned above, is of utmost importance when determining the meaning in the use of these terms. Because these terms are often used incorrectly, the context is usually the most reliable information to be used to determine the real meaning of what is being said.
I hope this has been helpful!
Laz