A eukaryote is an organism whose cells contain a nucleus within a membrane. The genetic material and information of a eukaryote is contained within this nucleus.
Eukaryotes vary from single-celled organisms to complex multicellular animals and plants. In fact, most living things are eukaryotes, made up of cells with distinct nuclei and chromosomes that contain their DNA. The only organisms that aren't eukaryotes are bacteria and archaea, known as prokaryotes. The word eukaryote comes from the Greek eu, "well," and karyon, "nut or kernel," which is a common scientific word-forming element that's used to talk about the nuclei of cells.