The Pentium M brand refers to a family of mobile single-core microprocessors and forming a part of the Intel Centrino platform.  A microprocessor incorporates most or all of the functions of a computer’s central processing unit (CPU).  The Pentium M processors had a maximum of 5–27 W depending on the model, and were intended for use in laptops.  The first Pentium M–branded CPU, code-named Banias, was followed by Dothan. The Pentium M-branded processors were succeeded by the Core-branded dual-core mobile Yonah CPU with a modified microarchitecture.  The Pentium M represented a new and radical departure for Intel, as it was not a low-power version of the desktop-oriented Pentium 4, but instead a heavily modified version of the Pentium III Tualatin design (itself based on the Pentium Pro core design). It is optimized for power efficiency, a vital characteristic for extending notebook computer battery life. Running with very low average power consumption and much lower heat output than desktop processors, the Pentium M runs at a lower clock speed than the laptop version of the Pentium 4 (The Pentium 4-Mobile, or P4-M), but with similar performance - a 1.6 GHz Pentium M can typically attain or even surpass the performance of a 2.4 GHz Pentium 4-M.  The Pentium M coupled the execution core of the Pentium III with a Pentium 4 compatible bus interface, an improved instruction decoding/issuing front end, improved branch prediction, SSE2 support, and a much larger cache. The usually power-hungry secondary cache uses an access method which only switches on the portion being accessed. The main intention behind the large cache was to keep a decent-sized portion of it still available to the processor even when most of the L2 cache was switched off, but its size led to a welcome improvement in performance.
The Intel Pentium M processor is a product born out of necessity. In the notebook, consumers quickly discovered that the Intel Pentium 4 processor simply ran too hot, consumed too much power, and subsequently sucked batteries dry while making a racket with its critical cooling system. AMD on the other hand had the mobile Athlon 64, and later the mobile Turion processor which easily outclassed the Intel Pentium 4 in every which way.  The Intel Pentium M processor easily holds its own against the mobile Athlon64 in terms of performance and its trump card is its extremely low energy usage. The Pentium M is based on an older P6 architecture, the same key design elements which in fact powered Intel processors from the venerable Pentium Pro right through to the Pentium III. Intel reverted back to this tried and true processor core architecture because it was more efficient than the Pentium 4 NetBurst architecture.