When comparing CPU power usage, most users will talk about the TDP, also known as Thermal Design Power. For example, the 89W versions of the AMD Athlon 64 X2 are compared with the 65W versions of the Intel Core 2 Duo. A logical conclusion would be to say that Core 2 Duo consumes less power. But in reality, there are too many problems with this logic to be true.

TDP != power usage
First, there is no direct relation between TDP and power usage. You see, TDP is a value intended for system integrators indicating the heat that the casing and ventilation should be able to handle, rather than the actual power usage. For example, three Athlon processors might share a 89W TDP label but actually have varying levels of power consumption. TDP does not equal power usage. It's simply a message to integrators selecting the CPU's for their casings tellig them the what heat the casing should be able to handle. Since nearly 100% of the power usage is converted into heat, the TDP value might indeed dictate the power consumption. But in reality, often the CPU uses less than the specified TDP value and the power consumption differs between models sharing the same TDP label. One could say however that TDP is the maximum dissipated heat that the casing should be designed for. This, however, is no equivalent to power consumption.

Stressed versus Idle
While power consumption is often measured when the CPU is working at maximum FPU/ALU stress (100% utilization), most CPUs are running idle or near idle most of the time. In these cases the maximum power usage is not the most important factor when considering power usage. Instead, the power usage when running idle is. Fortunately, AMD offers excellent power consumption levels when running idle. Think about it, would you rather have a CPU that consumes 5W idle and 90W stressed or 40W idle and 60W stressed? I'll take the 90W one.

AMD versus Intel
To make things worse, both AMD and Intel calculate the TDP value in a different way. Where AMD specifies the maximum power drain, Intel specifies a calculated value below the maximum. This means that Intel's processors could - at least in theory - consume more than the TDP value indicates. To 'convert' Intel TDP to AMD TDP you need to add about 17,5% to Intel's number. Intel has commented on this:

Did we forgot something?
Oh yes, AMD also features an integrated memory controller, while on Intel platforms the memory controller is provided by the chipset (more specifically the north bridge). This further complicates a direct comparison of Intel chips and AMD chips.

Concluding
Do not take the TDP literally! Instead, look at hardware reviews testing actual power usage, with power saving technologies such as Cool'N'Quiet and EIST enabled. TDP was never meant to indicate power usage.