You're probably familiar with grammatical word classes such as noun, verb, and preposition. These vary between languages – Japanese word classes are somewhat different than those in English, but the differences are not difficult to understand.
This page is just a brief overview of the major word classes in Japanese. Links are provided to pages from the Beginning Lessons covering each class in greater detail.
One preliminary concept you should know about is inflection, which is simply a sound change with a grammatical basis. Inflection of verbs and verb-like classes is called conjugation, and tends to be more involved than inflection of other classes.
For a couple of English examples, "car" becomes "cars" to show that it is plural, and "watch" becomes "watched" to show that it is past tense.
Languages differ in how much they rely on inflection rather than independent words to convey meaning. English has very little inflection compared to other European languages, whereas Japanese has no inflection in its nouns and quite a lot in its verbs and adjectives.
Japanese nouns have no inflection, and the few issues that English speakers may have with them are easy to overcome.
A few points about Japanese nouns were covered in The Structure of a Japanese Sentence.
You can more about nouns and their relatives, the pronouns, in Nouns, Pronouns, and Plurals.
Another group of pronouns and determiners are covered in Demonstratives: the Ko-so-a-do Series.
Japanese counter words will be covered in a future lesson.
Japanese actually has several distinct classes of words equivalent to English adjectives: the i-adjectives, which behave like verbs, the na-adjectives and no-adjectives, which conjugate using the copula, and attributives, which can be used only as noun modifiers.
Despite these complications, adjectives are used in much the same way in Japanese as they are in English: to give properties to nouns.
All four classes are introduced in Japanese Adjectives, which discusses both the modifier and predicate functions of adjectives, and also introduces the negative form.
Adjective conjugation will be discussed in more detail in a future lesson.
Japanese verb conjugation is much more extensive than in English, although in a way that is somewhat less complicated than European languages such as Spanish and French – none of this "indicative" and "subjunctive" nonsense, and no need to worry about "agreement" with gender and number either.
Almost all Japanese verbs fall into one of two regular conjugation schemes. There are also two notable irregular verbs in modern Japanese: suru "to do" and kuru "to come". You heard it: two. (You may sing your praises now.)
The way Japanese verbs are used in terms of word order was covered in The Structure of a Japanese Sentence.
The Japanese Copula is covered in The Copula "Desu".
Verbs and verb conjugation are introduced in Japanese Verbs and Conjugation.
Particles were introduced in The Structure of a Japanese Sentence, where we talked about the subject and object markers "ga" and "o". This group of particles are the case marking particles, the Japanese way of marking grammatical function, English has case marking only in pronouns: this is the difference between "I" and "me", "we" and "us", "he" and "him", and so on.
Most other Japanese particles work as postpositions, analogous to English prepositions. So the Japanese equivalents of in, at, of, to, from, and with all follow the nouns they are attached to. Other particles cover the functions of English conjunctions. Some particles, like に "ni" and で "de" have multiple uses. The are also compound particles, which may also include a normal noun as one of their parts (ex. ～のうえに/no ue ni "above"). There are a couple dozen very common particles that you will learn early on as you study Japanese.
Japanese also has adverbs, conjunctions and interjections, but there isn't much to say about these classes, so they will simply be explained as they come up in the lessons.