Grammatical particles are words that have no lexical value on their own, but associate with other words to mark functions such as case, mood, or tense.
Some linguists define particles as words that cannot be inflected, and by that definition many Western languages such as English and German can be said to have particles. However, they typically are used to refer to classes of words found in Eastern languages such as Chinese, Korean and Japanese.
In languages that extensively use particles, such as English, they can be thought of as replacing prepositions and articles, though they typically have a even wider role. Tagging entire sentences as interrogative or emphatic for example, is also a role they take on.