word is a positional attribute. It is short for word form and refers to one of the word forms that a lemma can take, e.g. the lemma go can take these word forms go, went, gone, goes, going.
A list of words is a list where each of go, went, gone, goes, going is listed separately and their frequencies are also calculated separately.
A search using words is a search which will only find exactly what was typed. The other word forms will not be included.
Word is case sensitive. apple and Apple are two different word forms.