For about 500 years ago, in the Netherlands and Germany they spoke different dialects and those dialects did not differ that much. For example the dialect in the east of the Netherlands was close the dialect in the north west of Germany. So the languages have a common root, and the division into 2 languages has been made historically not so long ago. Hence, the languages have a whole lot in common. What are then the difference? Dutch has adopted more foreign words, mainly French and English. Dutch uses far less gender, and far less grammatical cases. Dutch does not use Konjuntiv. Many words are almost equal. They only differ in the accent. Some words have totally different meanings though. Bellen means to bark in German, the sound a dog makes, but in Dutch it means to ring, the sound of a telephone or doorbell.

There's a big difference, even though there are some similarities they are not mutually intelligible without some previous study of the two. The comparison is similar to Spanish and Italian or Hebrew and Arabic. German, Dutch, English are from the same root, but with major differences between them.

Incidentally, the language closest to English is spoken in the Netherlands, in the northern provinces. It's Frisian or Frysk. I've looked at it though, and I can't really understand it. There is probably much less influence from French, Latin, Greek, etc.

From my own observation, they have a different morphology. When I listen to the Dutch language, there is a sound I often hear that sounds as if it comes from the throat. A guttural sound every 17 seconds or so. Based upon my knowledge, the spellings rules (orthography) are also very different.