Illustrated Glossary of Organic Chemistry

Electron withdrawing group (EWG): An atom or group that draws electron density from neighboring atoms towards itself, usually by resonance or inductive effects.

Trifluoroacetate ion is a weaker base than acetate ion because the trifluoromethyl group is attracting electron density away from the carboxylate. The trifluoromethyl group is an electron withdrawing group:
Acetate ion   
Trifluoroacetate ion

Aniline is a weaker base than ammonia because the benzene ring is attracting electron density away from the nitrogen lone pair by resonance. The benzene ring is an electron withdrawing group:

  
In ammonia the lone pair is
localized on the nitrogen atom.
In aniline, nitrogen's lone pair is delocalized by
resonance, as demonstrated by this resonance hybrid.