As others have said, the 'b' stands for 'baseband'. However the existing explanations aren't clear so I'll add some info. There are two ways to transmit information over wires: baseband and broadband. An example of broadband is (analog) cable TV: all the channels are combined together on one wire at once. The receiver selects different parts of the transmission spectrum to select different channels. You could receive every channel at once if your receiver were designed that way.
With baseband, only one signal is sent over the wire. That's simpler for digital transmission because the receiver doesn't have to do all the work of selecting 'channels' - there's just one. That channel contains the digital information (signal modulations) that are processed by the receiver into a data stream.
Obviously that's a greatly simplified answer, but that is the gist of baseband vs. broadband.
"b" is "Base", "T" means twisted pair, and "x" differentiates from 100bT (they have different frequencies). http://en.wikipedia.org/wiki/Ethernet_over_twisted_pair has some details.
"Base", in turn, is short for "Baseband", which means the signal is the only one on the wire at a time.
As others have said, the 'b' stands for 'baseband'. However the existing explanations aren't clear so I'll add some info. There are two ways to transmit information over wires: baseband and broadband. An example of broadband is (analog) cable TV: all the channels are combined together on one wire at once. The receiver selects different parts of the transmission spectrum to select different channels. You could receive every channel at once if your receiver were designed that way.
With baseband, only one signal is sent over the wire. That's simpler for digital transmission because the receiver doesn't have to do all the work of selecting 'channels' - there's just one. That channel contains the digital information (signal modulations) that are processed by the receiver into a data stream.
Obviously that's a greatly simplified answer, but that is the gist of baseband vs. broadband.
It stands for the type of base transmission.
http://en.wikipedia.org/wiki/Fast_Ethernet
The b is 'base', short for baseband. T is 'twisted pair'. X is just an identifier to distinguish this from other standards.