possessive particle
The particle の marks possession or attribution, linking a noun to another noun.
の is the possessive particle. It connects two nouns, where the first noun owns, belongs to, or is associated with the second. For example, わたしの本 means 'my book', where わたし (I) owns 本 (book). The order is always possessor + の + possessed item.
You can also use の to describe a characteristic or origin, like 日本のアニメ (Japanese anime) meaning anime from Japan, or 学生のせい (student's fault). It's very common and essential for basic sentence building. Note that unlike English 's, の is placed between the two nouns without any spaces or changes.
In questions like これはだれのペンですか (Whose pen is this?), の follows the question word だれ. You can answer たなかさんのです, where の stands in for the noun already mentioned, meaning 'Tanaka-san's (pen)'. This usage is natural in conversation.