An IELTS essay is a type of academic writing task that assesses a test taker's ability to communicate their ideas and arguments effectively in English. It is specifically designed to test the language skills of non-native speakers of English who wish to study or work in an English-speaking country.

On the other hand, an ordinary essay is a general term for any type of written composition that presents an argument or idea on a particular topic. It can be written for various purposes, such as to inform, persuade, or entertain the reader.

The main difference between an IELTS essay and an ordinary essay is the purpose and audience. While an ordinary essay can be written for any audience and purpose, an IELTS essay is designed to meet specific criteria and target a particular audience of test takers.

Another difference is that IELTS essays require a higher level of language proficiency and adherence to specific writing conventions, such as using formal language, organizing the essay into paragraphs, and following a specific structure (introduction, body, conclusion). In contrast, an ordinary essay may not have such strict requirements and may allow for more creative freedom in terms of language use and structure.

In summary, the main differences between an IELTS essay and an ordinary essay are their purpose, audience, and level of language proficiency required.