A word family is the base form of a word plus its inflected forms made with affixes.
A base word and its related forms share the same basic meaning. A learner needs to understand both the base word and the affix.
References
↑Hirsh D. & Nation P. 1992. What vocabulary size is needed to read unsimplified texts for pleasure?. Reading in a Foreign Language. 8. 689–696 [permanent dead link]