can anyone explain me the difference between baud rate and bits/sec?
One has to do with the signaling waveform (baud), while the other has to do with the actual data transmitted (bps). Through various modulation methods (QAM, for instance), one can encode multiple bits of information in one waveform shift. So, in the context of say a modem, the "baud rate" has to do with the waveform that the modem places on the phone line, whereas the bps has to do with how much data is actually being transmitted. You should probably note, however, that the term baud rate has been so misused as to make it nearly impossible to tell which meaning someone meant when reading a given piece of technical literature or documentation.
Jay's definition is correct. A "baud" is technically one change in signal state per second. If the receiver can discriminate between exactly two states, then you can send one bit per baud. If the receiver can discriminate between more than two states, then you can send multiple bits per baud. For example, the old Bell 103 modem standard switched between two frequencies (FSK), changing frequencies 300 times per second. Thus, you could transmit 300 bits per second with this 300 baud modulation scheme. The Bell 212 standard used 4-way phase modulation (QPSK), and could thus transmit 2 bits per baud. This modulation scheme is 1200 bps at 600 baud. The V.22bis scheme used quadrature amplitude modulation (QAM) with four phase angles and four amplitudes, for a total of 16 possible signals, and thus carried 4 bits per baud. Operating at 600 baud, it was a 2400 bps signalling scheme. The V.32 spec was 16-QAM at 2400 baud, for 9600 bps. As it happened, most people ignored this subtle distinction, and just used the word "baud" as though it meant "bits per second". It used to be quite common to hear people refer to their "2400 baud" or "9600 baud" modems, when they really meant bps. Only the most pedantic or technical discussions insisted on proper use of the term. I suspect that the SI (metric) penchant for naming every combination of units after yet another another scientist added to the confusion. If a Newton is shorthand for a meter-kilogram per second-squared, and a Joule is a Newton-meter and a Watt is a Joule per second, and a Volt(a) is a Watt per Ampere, you can see how people might have formed the mis-impression that Baud(ot) was a short name for a bit per second, especially given early signalling schemes that could only discrimate 1 bit per baud, and so where the "baud rate" was indeed the same as bits per second. It's very rare for me to see the term "baud" outside of the context of analog modems. These days, people usually refer to the speed of a link in bps, and might occasionally refer to the modulation scheme when necessary. For example, people will usually say that a DOCSIS cable modem transmits at 30 Mbps downstream. If you look, you'll find a reference to 64-QAM (6 bits/baud) at a baud rate of 5.056941M symbols/second. But nobody talks about a "five megabaud cable modem" or even incorrectly as a "30 megabaud" cable modem. We've learned our lesson, and the term is just "bps" now.