"What is often advertised as "high-speed dial-up Internet" or "accelerated dial-up" by service providers such as Earthlink and NetZero in the United States is a form of dial-up access that utilizes the newer modem standard v.92 to shorten the log-on (or handshake) process, and then once a connection has been established the provider will selectively compress, filter, and cache data being sent to the users home with the overall effect of increasing the speed of browsing most standard web pages (see also proxy server).

The term high speed is misleading as these processes do not increase the overall throughput of the line, only making more efficient use of the bandwidth that is already there. Certain applications cannot be accelerated, such as SHTTP, streaming media, or file transfers. The compression of certain files such as pictures can have a negative effect on the browsing experience of the user, due to the lower quality that it imposes." (Wikipedia)

Sources & Additional Information:
Wikipedia