The first flight of a new aircraft type refers to the first time the aircraft is flying during the development and testing phase. If the testing goes smoothly, it might be that no more changes are necessary and the aircraft can be delivered to customers as it is. But if problems show up during these test flights, changes might be necessary before finishing the final design.
The introduction refers to the first flight by a customer (an airline for commercial airliners or a military unit for military aircraft). This is also referred to as entering service. This could happen in the same year as the first flight (e.g. Boeing 737 Classic series in 1984) or many years later (e.g. Concorde in 1976, 7 years after the first flight in 1969).
Another term commonly used is the launch of an aircraft type, which refers to the start of the development program.
The IPTN N-250 you mentioned was never introduced into service. Wikipedia claims the 15 June 1997 as introduction date, but on this day the aircraft was shown at the Paris Air Show:
(image source: airliners.net, taken by Peter Vercruijsse in Paris - Le Bourget on June 15, 1997)
The image description says:
"Gatotkoco", the prototype N-250 returning from a demonstration flight at the Paris Air Show. The N-250 programme is currently (early 2000) on hold due to a lack of finance.
This does not count as introduction and is therefore a mistake in the Wikipedia article.