Contrary to popular belief, a Flemish language as such doesn't exist: there are however variants of the Dutch language spoken in Belgium; and these are, mainly for political reasons, sometimes referred to as "Flemish".
One of these variants is the standard Dutch spoken in Belgium, which is slightly different from the standard in The Netherlands in vocabulary and idiom. This southern standard is largely from Brabantic origin.
The rest consists of the Dutch dialects spoken in Belgium. These "Flemish dialects", as they are often called by the layman, do not form a unity however: i.e. they are not more closely related to each other than to the dialects spoken in The Netherlands.
Finally, however, there are among these also real Flemish dialects in the linguistic sense, that are spoken in the old county of Flanders (about a third of the Dutch-speaking region in Belgium), among which the most deviant is West Flemish.