Proto-Indo-European word classes, i.e. types of words, are defined on the one hand by the kinds of morphological forms (inflections or lack thereof) that are added to their roots, and on the other hand by inherent grammatical functions that determine how they combine (syntactically) with other words in a sentence.
Nouns and pronouns were inflected for case and number, and nouns and adjectives additionally for "gender" or class, while finite verbs were inflected for person and number endings with suffixes on root and stem forms that might differ depending on categories of tense, aspect, mood, and voice.
Besides the inflected word classes (nouns, pronouns, adjectives, and verbs), small words ("particles") were uninflected; these were words, such as adverbs and preverbs, that typically added grammatical and pragmatic meaning. As in other languages, particles (a.k.a. grammatical words) signal cohesive relations in a sentence and sometimes have functions that inflectional endings also have. In older IE inflectional endings, independent "ad-verbs," "ad-positions," or particles often marked relations between nouns and verbs. In some older IE languages, ad-positions as "postpositions" followed a noun that had some spatial or temporal relation to the verb ('go town to', 'go Monday on'), by contrast with "prepositional" uses in English ('go to town', 'go on Monday'). Such grammatical words did not have suffixes or inflectional endings. Adverbial particles might also indicate time ('today'), mood or modality ('if'), or aspect.
Chronological issues of subgrouping deal with exactly what belonged to Proto-Indo-European (or stages of it) vs. what constituted an innovation in one or more branches.