Consumers have been used to measuring CPU/processor speed in clock speed: megahertz and gigahertz. The clock speed or rate is how many times the processor does its basic operations: one hertz is one per second, so a gigahertz is 1 Trillion (or 10 raised to the 9th power) per second. AMD stopped this practice when it claimed that it can deliver “equivalent” performance to Intel CPU’s at lower clock speeds. It came up with a number called a PR Rating. For instance, the Athlon XP 2200 runs at a clock rate of 1.5 Ghz.
Intel has switched to a family and processor number scheme, where the number is a model/brand number considering the features that impact performance: clock speed, cache size – high speed memory used to handle data coming in from the slower main memory – and now number of cores, or actual processors on a single chip.
This scheme is most important in mobile chips. The old Intel “higher megahertz is faster” thinking which shaped the Pentium 4 product design made the average computer user or buyer think the same. When they came up with the Pentium M, based on the Pentium 3 Mobile architecture, the numbers went down to the low 1+ Ghz while keeping performance equivalent to the mainstream Pentium 4. Buyers had to be told some other way that this was a superior item to buy despite the low megahertz. Thus the “Centrino” brand. Centrino is different from “Celeron”, and is not a processor. It’s a brand for laptop makers who use the Pentium M CPU, an Intel mobile chipset (the supporting minor CPUs on the motherboard), and an Intel wireless chip for 802.11 WiFi.
The “Celeron” is Intel’s generic term for the budget CPU. The “Celeron M” based on the Pentium M architecture is of a different design from the desktop “Celeron D.” Both are good CPUs but are tarnished from the old reputation of underperforming Celerons. The Celeron M has half the L2 cache of the Pentium M it was based on – there are two kinds now, “Banias” and “Dothan”, places in Israel where the “M” architecture was developed. It also has reduced power saving technology. Coupled with the budget laptop manufacturers decision to include fewer cells in their batteries (4 vs 8), leads to the Celeron M being optimized for “mostly plugged” operation.
Otherwise, many Celeron M units are similar to their Pentium M/Centrino siblings. For instance, within the IBM Thinkpad R50 1834 series, the MA4 is a Celeron M unit with an Intel 855GM chipset and Intel 2200 Wireless card. If the Celeron M were upgraded to a Pentium M, it “could” be a Centrino.