The exercises, drills, and solutions are curated from a collection of resources available on the MIT Japanese 2 course site. OCW is not hosting all of the exercises due to audio or visual components that are incompatible with our license.
Solution files provide the solutions. Early in the course they include kanji, kana, furigana, romaji, and English translations. Later in the course, not all of these are provided.
Kanji are the characters used to write Japanese. They can be read using one or more phonemes. Individually and in conjunction, they can represent different words or concepts. Frequently, a given kanji is used in many cases and can have many pronunciations—sometimes even in the same word. Example: 日 by itself is pronounced hi, and means "sun" or "day." In words like 日曜日, the same kanji is read as nichi and bi. Nichiyoubi means "Sunday."
Kana is the Japanese equivalent of the Roman alphabet, representing individual phonemes. There is both hiragana and katakana. The former is used for Japanese words, and the latter is used for cognates, or borrowed words.
OCW has published a supplemental collection of Kana resources (hiragana and katakana), contributed by various MIT faculty. While not used specifically by students enrolled in 21G.502, OCW visitors may find it a useful complement to the 21G.502 materials presented here.
Furigana is a pronunciation guide for kanji. It is written above kanji. Example: 
Romaji is the Roman alphabet when used to write Japanese phonetically.