The chips previously codenamed Silverthorne and Diamondville are based on an entirely new architecture and have been designed from the ground up to consume as little power as possible. Intel plans to target two main markets: with Silverthorne they hope to get inside the so-called 'Mobile Internet Devices' (MIDs). The more powerful and a bit more power hungry Diamondville will be targeted at small form factor laptops like the Asus Eee PC.
The Atom will be manufactured on an 45nm process and will carry a TDP specification in the 0.6-2.5 watt range - which is minuscule compared to a mobile Core 2 Duo's 35w TDP. Though comparing these CPUs' performance output would make no sense at all since they are aimed at completely different markets, the Atom is still a fully working x86 processor. To give you a better idea of how small these CPUs are, here's a promotional video from the Intel Mobility team blog.