A word form (known as a word in corpus terminology) is a unit which remains morphologically (and possibly also orthographically) specific. With its generality it stands between a token and a lemma.
While a token is one specific realization of a given unit, a word form is a standardized unit; a type. E.g. the word form eat can have a great number of different realizations(tokens).
On the other hand a lemma is a unit on yet a higher level of abstraction, because it disregards morphological and orthographic characteristics. The word forms eat, eats, ate, eaten have the same lemma eat. Additionally, on the level of word forms, most approaches differentiate between lower- and upper-case letters (the forms eats, Eats a EATS are considered to be different word forms). Unlike a lemma, which can be understood as a set of different forms, a word is a single form of a given word.