NEW ONES LIKE THE CORE 2 QUAD Q6600 SUPPORT SPEED STEP TECHNOLOGY

T4200 has Enhanced Intel® Speedstep Technology which the celeron doesn't.

http://ark.intel.com/Product.aspx?id=37251 vs http://ark.intel.com/Product.aspx?id=41498

Not having speedstep on an laptop will be bad for your battery consumption.

Newer Intel processors incorporate something known as "SpeedStep." When the processor is not being used extensively, it downclocks itself to conserve power. You can disable this feature if it bothers you in your BIOS.

In general, underclocking.

Some specific types of underclocking include "dynamic frequency scaling", "CPU throttling", "SpeedStep ", "Cool'n'Quiet ", "PowerNow!", "Downclocking", "LongRun" and "LongHaul".

Hyper-Threading is a technology used by some Intel microprocessors that allows a single microprocessor to act like two separate processors to the operating system and the application programs that use it.