How does astrometry differ from the doppler effect?
Astrometry is measurement of positions of astronomical objects whereas the Doppler effect is one of the key tools used in Astrometry.
Astrometry concerns the measurement of positions and velocities of astronomical objects.
The Doppler effect, named after the physicist Christian Doppler, is one of the tools used in astrometry.
For example, different elements cause characteristic emission or absorption lines in the spectra of stars or galaxies, but we observe these lines shifted towards higher (blue) or (more commonly) lower (red) ends of the spectrum. This frequency shift more-or-less corresponds to the velocity with which the observed object is moving towards or away from us. The spectra of objects is also red shifted by strong gravitational fields.
Due to the form of expansion of the universe, objects which are further away are also moving away from us faster and are thus generally more red shifted. So red shift also allows us to get at least a rough idea of how far away objects are too.