It is just gender. Like: Male, female, neuter. Do you speak any other language? It is about the same as the difference between: 'der, die, das' in German. Or 'le, la' in French.

It is less frequently used than in German, since we got half way abolishing gender. But unlike the British, we did not fully succeed in this. DE is actually the female and male form of the article merged together, and HET is the neuter form.

Indeed you need to memorize a lot to sound natural, sorry! People will understand you if you always use "de" but it'll sound weird.

As a hint for learning this, try associating a color with "het" and another color with "de" (by writing all "het" and "de" words in their respective color). It's helped me memorize German der/die/das words and Chinese tones more easily.

Het en de

Wanneer kan ik ze op het juiste moment gebruiken?

Bestaat/Is er een verschil?

Lot`s of language information can be found here:

http://beterspellen.nl/website/index.php?pag=242

Written in Dutch. Dutch teacher are getting information from this website. 

Well you just have to remember each word with its gender. There are not really definite rules to say which words have which gender.

Here are some rough general rules, but they do not always apply:

http://www.inventio.nl/genus/uitleg.html