Three basic characteristics differentiate microprocessors:

In both cases, the higher the value, the more powerful the CPU. For example, a 32-bit microprocessor that runs at 50MHz is more powerful than a 16-bit microprocessor that runs at 25MHz.

In addition to bandwidth and clock speed, microprocessors are classified as being either RISC (reduced instruction set computer) or CISC (complex instruction set computer).

See the Microprocessor Comparison Chart page in the Quick Reference section of Webopedia for a comparison of microprocessors.

PC system processor reference guide 
Comprehensive look at the system processor. Covers semiconductor technology, manufacturing, characteristics, power, voltage levels, cooling, packaging, sockets, external and internal architecture. Discusses performance factors. Complete look at over 20 popular processors, including the latest offerings from Intel, AMD and Cyrix.

VLSI Microprocessors 
Excellent collection of high performance microprocessor resources from the Supercomputer Software Department at the Russian Academy of Sciences.

National Semiconductor home page
This home page offers links to company and product information, design engineer resources, news articles, information on environmental health and safety, and an annual report.

Processor Architecture and Operation
Provides a complete look at the way processors work. Includes sections on external processor interfaces and operations, internal processor architecture and operations, and processor performance. This page is from "The PC Guide".

Processor Physical Characteristics
Discusses the physical properties of processors. Includes information on processor manufacturing, physical chip characteristics, power and voltage, cooling and packaging, and sockets and slots.

Yahoo!'s microprocessor page
Yahoo!'s directory of microprocessor hardware components.

Great Microprocessors of the Past and Present
Article on "Great Microprocessors of the Past and Present", including an index and in-depth descriptions.

Microprocessor Comparison Chart
A cut-to-the-chase guide that compares and contrasts popular processing chips.