You very well may have heard this piece before, as it's quite famous. You probably heard it attributed to the great 18th Century Venetian composer Tomaso Albinoni. That's the hoax. It's not baroque, it's fake neo-baroque written in the 20th Century by Albinoni's biographer Rimo Giazotto.
Giazotto was quite famous in musical circles, a professor at the University of Florence and nominated to the Accademia Nazionale di S. Cecilia, one of the oldest musical institutions in the world. He published this work which he said he discovered on a fragment of a manuscript and transcribed. But it was all made up - Giazotto composed the piece, but since he was an expert on Albinoni's style, he passed it off as real.
The piece has been very popular since its introduction, being used in almost two dozen films and a bunch of television shows (including Monty Python's "Funniest Joke In The World" sketch).