Mew is the secret hidden Pokemon from the original set of games. While the game touted a number of 150 different pokemon, Mew was the top-secret #151. Except despite a movie based on Mew (and Mewtwo) it was not available in the original games at first. I got one though with my Gameshark, but Nintendo did give it out a official pokemon events. Since it was actually in the game's memory, there were a few other ways to obtain Mr. Mew.
Despite it's "legendary" status, Mew isn't exactly the greatest pokemon. It's got some neat abilities, but for it's rarity it's a tad disappointing. Mewtwo is the better of the duo in my opinion.