Generally speaking, bandwidth of a channel is the maximum information carrying capacity of the channel.
A "signal" might have to be encoded or altered in order to carry it over a "channel". For example, you might start with an analog signal that represents human speech, truncate it at 8000 samples per second, process that 8 KHz signal through a processor at (say) 100 MHz, and then multiplex it with a whole bunch of other speech signals to transmit it over a 1 Gigabit/second fibre optic cable, possibly with error correction codes added. As there might be multiple layers of multiplexing along the way, there might be multiple layers of "channels" whose capacity could be measured, so when you see a figure such as a "56 Kbit/s channel", you should not (without further information) assume that it is a physically distinct channel of that speed, just that it is somehow getting sent through a process that promises that much transmission bandwidth for that logical channel.
