A creole is a language that comes from a simplified version of another language, or the mix of two or more languages. In Haiti, people speak a creole that's mostly based on French.
When a new language is a variation on one or more older languages, you can call it a creole. Many creoles arise from the contact between both languages and cultures — Haitian creole sounds similar to French, but it's also influenced by Portuguese, Spanish, and some West African languages, since it emerged from the connection between French colonists and African slaves. Creole comes from the Portuguese crioulo, "servant raised in one's house."