Induction is the redistribution of electron density through a traditional sigma bonded structure according to the electronegativity of the atoms involved. The inductive effect drops across every sigma bond involved limiting its effect to only a few bonds.
Conjugation is a redistribution of electron density similar to induction but transmitted through interconnected pi-bonds.  Conjugation is not only affected by electronegativity of the connected atoms but also affected by the position of electron lone pairs with respect to the pi-system.  Electronic effects can be transmitted throughout a pi-system allowing their influence to extend further than induction.
In the context of electronic redistribution, an electron-withdrawing group (EWG) draws electrons away from a reaction center. When this center is an electron rich carbanion or an alkoxide anion, the presence of the electron-withdrawing substituent has a stabilizing effect. Similarly, an electron-releasing group (ERG) or electron-donating group (EDG) releases electrons into a reaction center and as such stabilizes electron deficient carbocations.
In electrophilic aromatic substitution and nucleophilic aromatic substitution, substituents are divided into activating groups and deactivating groups. Resonance electron-releasing groups are classed as activating, while Resonance electron-withdrawing groups are classed as deactivating.