From wiki...
The Persians are an Iranic ethnic group who speak the Persian language and share a common culture and history.
Persians are the main ethnic group of Iran and are the majority and dominant ethnic group of Iran
The term Persia was adopted by all western languages through the Greeks and was used as an official name for Iran by the West until 1935. Due to that label, all Iranians were considered Persian. Therefore, many Western sources, when regarding Iran's history, will label many non-Persian Iranians as Persians, which distinguishes nationality, not necessarily the Persian ethnic group. Also, many others who embraced the Persian language and culture are also often referred to as Persian, not necessarily meaning ethnic group, but rather as a part of Persian civilization (culturally and linguistically).
http://en.wikipedia.org/wiki/Persian_people