Core2Duo and Core2Quad are 2 and 4 core processors based on a particular microarchitecture (most of them Penryn and Wolfdale). The i5s and i7s are the replacement processors, based on a newer archicture. Basically the i5s and i7s are more powerful. Last time I spoke to an intel rep he said the new architecture would be Intel’s primary platform heading into the future.
The i5s and i7s at the moment are 4 core processors. There will be dual core i5s released soon (in Australia that is). The core2duo and core2quad models will become low cost entry level processors, like the Celeron is at the moment. Celeron and Pentium and the moment are used to denote cheap, entry level processors. The Atom range are low power processors designed for netbooks.
You are correct, the range at the moment is very confusing if you haven’t been following it closely, and the best advice is to read CPU benchmarks from the links already given. Not much can be predicted about a processors performance by simpling looking at processor specifications, for example clock speed. I still see people who don’t understand a 1.6Ghz i7m is vastly superior to a 3.0 Ghz Pentium D, even though the clock speed is half. Since multi-core CPUs came out the clock speed taken alone gives almost no indication of performance. Even the number of cores doesn’t give much indication, as witnessed by the benchmark scores of i7s and core2quads. A benchmark is the only reliable way to predict CPU performance.