What are categorical variables, dummy variables and one-hot-encoding? What is a dummy trap?

Categorical Variable: A column containing more categories

Dummy Variable: When you create a column for each category that you have

One-hot-encoding: Is encoding the dummy variables with ones and zeros.

Dummy Trap: Encoding all the dummy variables leads to unnecessary or incorrect computation: if you say "yes" it's implicit that you don't say "no".

If the country it's not Spain, nor Italy, nor USA, not Saudi Arabia, by exclusion I know it's China!
In the same way we don't write a Dummy Variable for the last category [country] (China).