Modem speeds are typically measured in bits per second. For dial-up modems, due to the technology used to deliver normal phone service, the fastest rate available is 56 kilobits (one thousand bits) per second. Digitized information, be it sound, video, or data files are converted into a series of ones and zeros in order to be understood and interpreted by your computer. Your modem converts digital information into analog signals for transmission through the phone network, and from analog to digital from the phone network to your computer.
Over time there have been various standards developed to support 56k data transmissions; X2 and KFlex were early, non-standard technologies (developed by modem manufacturers), while V.90 and the current V.92 are industry standards (used by all modem manufacturers to assure compatibility).
Sending and receiving signals through the phone network requires electrical current. In the data communications world, the more information (bits per second) the more current you need. If you apply too much current, there is a greater chance of the network developing a problem called crosstalk, or the unintended leaking of signals from one phone line to another (if you've ever heard another conversation on your telephone then you are familiar with this phenomenon). The FCC (Federal Communications Commission), with that problem in mind, decided to reduce the range of current available for phone company equipment to use, and thus reduced the speed available to analog modems. So while the modem may be capable of 56K, to help prevent crosstalk the FCC limits the amount of current that phone companies can use to send signals over the network, limiting data throughput to a maximum of 53 kbps.