That is actually a misconception. Manga is usually the basis for most anime out there. Many anime series are developed using the plotline and characters from the manga, usually with some edits involded. These edits sometimes add new characters, change the plot, or make it so that the entire plot can fit within a nice 26-52 range of episodes.
Manga is the japaneese form of graphic novels. There is also Manwha, which is the korean form, but that is not concernable right now. Manga is created in a right to left format, instead of a left to right format, and each page is split into panels [I wonder why I said that part ... it seems obvious.]
Manga has much more forms of art than anime does, mostly because most anime is done by companies, whereas each manga is written by a different person, each of them called Mangakas. There are some cases where a manga is drawn by a group of people, where each of them does a certain part of the drawing process, such as one person does characters, another does backgrounds, and such like that.
Occasionally, two people write a manga. One of them is the plot 'director', while the other one draws. This happens quite often in fact - two popular mangas, Death Note and Hikaru no Go, were both drawn by Takeshi Obata, though written by different people.
A flashback page from Death Note. Many flashback pages are colored in black instead of the usual white to show that they are indeed flashbacks. Notice how the use of contrast between the light and dark are used to give the characters a more evil look than they would normally have.
A scene from Hanazaki Kimitachi [For You in Full Bloom], a shojo manga. Notice how the style of the manga is different than the ones before and after. This form is typical for most shojo mangas. Notice the large kanji behind the girl in ithe last panel. This is common in most mangas, and are called sfx [special effects], and are used to show sound or emotion and the like.
A two page scene from Flame of Recca. This shwos a more free-form panel distribution, with panels just floating in the page. In the right, it is up to the reader to distinguish the 'panels' for him/her self.
A page from Ai Kora [Love College]. Note how in the first panel, there are lines. These lines are called action lines and are used to focus in on one thing in the panel. Additionally, notice how in the last two panels, the guy suddenly 'chibifies', and motions are greately exagerated. These are used as a form of comedy.
An action scene from the manga Psyren. Note how the Action Lines, are used to show motion in fights, as well as focus on one thing as shown above.
The title page for Full Metal Alchemist. Most manga chapters start with something like this.
The previous pages are some good examples of the styles and artwork associated with manga.
Manga can be divided up into the same way that Anime is - shonen, shojo, romance, action, etcetera. Many people enjoy only a certain type, but often times the types are intertwined - you will find action mangas with some romance themes in them. The same is true for anime.
There are many more mangas than animes.
Manga is usually distributed in magazines, that only contain one 'chapter' of a manga. These chapters, if the series is good enough, will be published in volumes, or tanboken as they're known in japan. Mangas can run for various amounts of time and chapters, and are published every week, every two weeks, or every month.
I hope this has helped you a bit on what manga is.