"HTTP is the application level protocol providing basic request/response semantics. While we believe that there may be opportunities to improve latency at the transport layer, our initial investigations have focussed on the application layer, HTTP.For prototype, they have created an open source web server and an SPDY-enabled Chrome browser. In lab tests, they have observed up to 64% reductions in page load times in SPDY over HTTP.
Unfortunately, HTTP was not particularly designed for latency. Furthermore, the web pages transmitted today are significantly different from web pages 10 years ago such and demand improvements to HTTP that could not have been anticipated when HTTP was developed..."
"SPDY replaces some parts of HTTP, but mostly augments it. At the highest level of the application layer, the request-response protocol remains the same. SPDY still uses HTTP methods, headers, and other semantics. But SPDY overrides other parts of the protocol, such as connection management and data transfer formats."
Links: SPDY Whitepaper | Let's make the web faster - a Google initative