this word is confusing me, i know its data rate that a wire can carry, but still i am confused about this concept. please explain it in more understandable form. Any answer in simple words would be preferd.
I think the best basic analogy for someone to understand is using water as metaphor. The amount of water that can be transmitted through your kitchen faucet is less than a firehose can push. The size of the pipes determine how much water can pass through. This is roughly equivalent (in layperson's terms) to the idea behind bandwidth.
Are you asking about the origin of the word ? If you, you need to dive a bit into signal theory.
Information is transmitted by modulating a signal. A signal is "something" that changes over time. You can describe it with all the values it takes at different times; or you can look at its Fourier transform, which gives you the spectrum of the signal. The spectrum tells you which frequencies the signal puts energy in.
When you send a constant signal, over a single frequency, its spectrum is a single vertical line. Also, since the signal is constant, it cannot transmit information.
When you modulate this signal at a certain rate (on/off, phase reversal, fsk, whatever), the spectrum becomes wider. That means the signal puts energy not only in its base frequency, but in the neighboring frequencies as well. (This is because modulation (multiplication) in the time domain corresponds to convolution in the frequency domain).
A range of frequencies is called a band. The bandwidth of a signal is then the width of its spectrum, or the range of frequencies it puts energy in. Bandwidth is linked with the theoretical maximum data rate of a signal: the faster you modulate, the wider the spectrum.
While the troughput of a signal gives you the rate at which you can transmit data with it, its bandwidth indicates how much of the spectrum your signal is occupying; this is important, since signals that have overlapping bands will interfere with each other.
Bandwidth is essentially a capacity. i.e. How much data you push can onto the pipe/cable per second. The bigger the pipe, the greater the bandwidth.
Bandwidth is not the only determining factor in determining a connection speed. You need to know the "Latency" as well. Latency is delay. i.e. How long does it take for a packet of information to go to a location and come back.
Here is an example of a system with High Bandwidth and High Latency:
Lets say you had a 2 GB flash drive tied to the foot of a pigeon. The pigeon delivers the flash drive to the end location and comes back in one hour.
The bandwidth of this system would be 2GB per second (assuming it took you a second to tie the flash drive to the pigeon), since you could push 2GB of data at once onto the pigeon. The Latency would be 1 hour. It is how long it took the pigeon to fly to the location and come back.
In essence a good connection will have high bandwidth and low latency.
Wikipedia (Simple) to the rescue!