Conroe = desktop Core 2
Woodcrest = server Core 2
Merom = mobile Core 2
The story goes like this: Intel created Netburst to push clockspeeds up through the roof. It was initially supposed to reach 5-6GHz, I think. Obviously, they never got past 4GHz stock. At the same time Intel's Netburst group was doing their thing (summary: the Prescott failure), the research team over in Israel came up with this nifty thing called the Pentium M. It dominated the notebook scene, and for good reason: it offered good performance with good battery life. Intel has basically scrapped their Netburst development, and Pentium M evolved into Yonah (current brand name: Core Duo). Yonah evolved into Conroe/Woodcrest/Merom (Core 2 Duo). So, here we have Conroe and co., which look like they've reclaimed the performance crown from AMD, both in outright performance and in power consumption.
They are all part of the same family, they share the same basic architecture, but there are some slight but significant differences between them. Conroe and Woodcrest are very similar. Woodcrest, being a server processor, is rated more conservatively, so it's more durable and less likely to fail (very important for server/enterprise).
Conroe is the flagship. It's what most of the hype is about.
Merom is the mobile version of Conroe. It uses something like half the power, but that comes at a speed penalty. It also supports much more advanced power-saving techniques (sleep states).
Woodcrest is all about performance-per-watt and reliability (this was previously where the Opteron was king).
Conroe is all about performance (this was previously AMD's territory).
Merom is all about battery life (Intel has always done well here, ever since the Pentium M and Centrino).