Basically, Flemish and Dutch are the same language. Especially in the written language, there are very few differences. However, when you look at colloquial texts, like articles in popular magazines or television series, the situation is rather different. Many different words and expressions are used. On television, for example, many drama series are subtitled if they are broadcast in the other region, because otherwise people cannot understand everything that is being said.

So, no matter how you put it, it cannot be denied that there are differences between the two varieties of Dutch. The most obvious difference between the two varieties is the pronunciation. This is probably most noticeable in the pronunciation of the letter 'g': the Dutch pronounce it with a rather harsh gliding sound in the throat, while the Flemish produce a softer sound. There are also differences in vocabulary: the American word for 'pavement' is 'sidewalk', likewise the Flemish often call a 'trottoir' a 'voetpad' (pavement). In some minor instances there are differences in grammar as well, especially in the order of verbs at the end of sentences. The spelling is exactly the same in both varieties.

In fact, both the Dutch and the Flemish speak variants of the same language. There are even variations within one country: the language in Amsterdam is as different from that in Eindhoven as Cockney is different from Geordie, and the sounds you hear in Bruges are as different from those in Brussels as the speech of Dallas is from that of New York. Probably even much more so. So, it may be impossible for a person from Antwerp to understand his co-citizen from Gent (who lives 80 kilometres away) if he's speaking in his own dialect and vice versa. However, if both the Dutch and the Flemish speak standard Dutch, they can understand each other perfectly.