It is important to understand the grammatical differences between these words. Clothes: This is a PLURAL word, referring in general to what we wear. It refers to shirts, trousers, dresses, socks, etc collectively. It has no singular form. Clothing: This is an UNCOUNTABLE word with a similar meaning to 'clothes', but it is less common. Garment: This is a COUNTABLE word referring to one single item of clothing. A shirt is a garment. Two shirts are garments. Costume: This is a COUNTABLE word to describe an entire outfit worn for a specific purpose. Nowadays, we use 'costume' for what people wear when they want to assume a different identity, and it can include wigs and make-up. For example, actors wear costumes on stage.

Clothes and clothing are used to generally talk about anything you wear on your body. Garment implies clothing worn for a special event, such as for some religious ceremony. Costume implies that the clothes are supposed to make you look like someone or something different, and may include things beyond clothes. Costume is also the word used to describe what children wear on Halloween.