"Flemish" technically has a different meaning from "Belgian Standard Dutch" — the latter being the standard form of the Dutch language as spoken in Belgium, much like how "Holland" is often used as a synonym for "The Netherlands", but technically only comprises a region thereof. One may view the dialect map of languages spoken around the Netherlands here. The parts called "Oost Vlaams" — Eastern Flemish, and "West Vlaams" — Western Flemish, in brown, as well as "Zeeuws Vlaams" — Zealandic Flemish, is where Flemish proper is spoken.
There is a big difference between "Standard Dutch" and the various regional dialects. Standard Dutch is for many speakers in the Randstad region of the Netherlands in fact their native language and dialect, as well as increasingly often in eastern and southern parts of the Netherlands; it is virtually never the native language in Belgium where speakers are generally brought up in local dialects but master the standard form at school. The different regional dialects of Dutch are not generally highly mutually intelligible, and as a native speaker of Standard Dutch, I have severe difficulties following many of the actual proper local dialects, as much as an average Canadian would have following say Yorkshire Dialect. Standard Dutch spoken by Belgians however has nigh identical grammar and vocabulary to Standard Dutch spoken by Dutchmen and there should be as few problems in communication as a Canadian speaking general American speaking with an Englishman speaking Received Pronunciation.
In Belgium itself there also exists something called "tussentaal" — in-between language, which could be considered Belgium's own form of "Standard Belgian Dutch", which is generally used among Belgians from different regions to communicate with one another, and often used on t.v. in informal contexts, whereas in formal contexts like newscasts Standard Dutch is used; this is markedly different from Standard Dutch in grammar and core functional vocabulary, but should be easily intelligible to most Dutchmen, partly because it is close enough to it, and partly because Dutchmen are used to it as they watch enough Belgian television.