There are two types of buses that carry data to and from a computer's CPU. They are the frontside bus and backside bus. Surprisingly, there is no correlation between these and the backside and frontside airs that snowboarders talk about.
While the frontside bus carries data between the CPU and memory, the backside bus transfers data to and from the computer's secondary cache. The secondary, or L2 cache stores frequently used functions and other data close to the processor. This allows the computer's CPU to work more efficiently since it can repeat processes faster.
When the processor needs information from the L2 cache, it is sent over the backside bus. Because this process needs to be extremely fast, the clock speed of the backside bus cannot afford to lag behind. For this reason, the backside bus is often as fast as the processor. The frontside bus, on the other hand, is typically half the speed of the processor or slower.