A logo is a picture or drawing that is used by a person, group, or company to mark who they are. A wordmark, or 'logotype', is a logo made entirely of a word or words. It is usually the name of a product or organization.
A company or group can use a 'logo' on the things that they make, like on the things they sell, on letters, and in advertisements. When a person sees the logo picture, that person knows that the thing they see the logo on came from that company.
Logos can have letters and words in them. Many logos have the name of the company or group in them.