Chinese Classifiers are "special characters" used along with numbers to define the quantity or with demonstratives to identify the object.

Classifiers don't have any meaning without the numeral or the demonstrative; they always need the combination of other words, such as nouns. If you combine a noun with a number or a demonstrative, you must use a classifier.

一个人

[yī gè rén]

One person

The classifier 个 [gè] is the most common one, and it can be used as a general classifier in Mandarin. However, most nouns have one or more particular classifiers associated with them, and you should use these instead of the general classifier. Depending on their properties, these classifiers can be used with different words: shape, size, quantity, type, etc.

一本书 [yī běn shū] One book

那辆车 [nà liàng chē] That car

You can see the classifiers for "book" and "car" in these examples. The first one, 本 [běn], is used for books and similar objects like magazines, notebooks, etc., as you might have guessed, objects with similar properties. The second one, 辆 [liàng], is used with almost any vehicle: car, van, bus, etc.