Manga is essentially the comic version of the anime...
In MY opinion, It's all of the storyline condensed in an easy to read format, where you can read a single episode (30min worth of show) in about 10 minutes, and be on to the next one.
BUT
A lot of the time the storylines for the anime and the manga differ...
And example of that would be Fullmetal Alchemist.
The original FMA was completely original, different than the manga, then when they finished the manga they came out with FMA Brotherhood, which was the anime that followed to the manga.