An "advertisement" is some way for a business to get customers, for a politician to get votes, for an organization to change people's opinions, etc.
Forms of advertising include:
- T.V. commercials
- internet advertisements
- printed advertisements in a newspaper
When people talk about advertisements, they usually use the casual word "ads":
I saw an ad for this cool gardening tool you might like.
You can use the full form "advertisement" in formal writing, formal speeches, or when you want to sound more intellectual.