A representative represents someone else in an official, public or legal way—that is, they are chosen to speak for or stand in for someone else in the government, in court, etc. A member of parliament or congress is often called a representative.
Mr. Jones was a representative of the water company. He came to our meeting to discuss the opinions and interests of the company's board with the town council.
In the US, the lower house of the legislature is called the House of Representatives. Representatives come from all over the US to speak for their voters' interests in Congress.
Representative can be used as an adjective to mean something is a good or typical example of a phenomenon or group.
The Beatles' Sargent Pepper's Lonely Hearts Club Band is quite representative of the psychedelic music of its era. The use of Indian instruments and sounds, the long digressions, and the strange lyrics were quite popular in the rock of the late 60s.
An ambassador is a type of diplomat who is chosen by the head of state, for example, a president, as an official representative of their own country in a foreign country.
When he was the American ambassador in Moscow, he often got into trouble for his tweets. Ambassadors are supposed to be very diplomatic and respectable, but he often used informal language.
Sandra is studying Chinese and international relations at university; I think she's going to be appointed ambassador to China someday.
An ambassador works in an embassy, or a special office where diplomats represent their country in a foreign capital city. The property of the embassy is usually considered the territory of the other country whose ambassador works there.
Ms. McMitchel started working in the embassy in Madrid several years ago, so her Spanish is quite good now. She's hoping to get transferred to a Latin American post next.
The travel agent told Sam that if he has any legal troubles, he should contact his country's embassy. They can provide legal advice for him.