Basically it's a remake of Half-Life. Since HL:Source didn't maximize the Source Engine potential, Black Mesa is needed to give players maximum experience. Using the Hammer and Source Engine, they made a remake of the HL map, not ported it, and also the textures. For sound they also made brand new voices. Last but not least, they made realistic and brand new physical movements/sequences.

Well, actually Black Mesa is free( Make a Steam account, download Source SDK (free) and then Black Mesa), so you don't have to buy a game in order to play Black Mesa.