What are the differences between a wavelength meter and a spectrometer?
Spectrometers typically have resolution on the order of 0.1 nm, whereas wavelength meters can have resolution up to the order of 0.0001 nm.
Spectrometers are typically smaller than wavelength meters, and are generally cheaper (due to the weaker resolution).
Another major difference between the two is that wavelength meters generally employ some sort of interferometer to distinguish absolute wavelength. Common interferometers used are the Michelson and the Fizeau.
Figure 1. Michelson Interferometer
Figure 2. Fizeau Interferometer
Wavelength meters utilizing the Michelson interferometer must have a reference laser (wavelength known accurately) built-in for calibration. The Fizeau models have no moving parts, and are solid state devices. Fizeau models are currently the most accurate wavelength meters available.