Het = neuter definite article (the). It is cognate with English 'it' which was 'hit' in Old English, however whilst Old English had 'hit' as a pronoun and 'þæt' (that) as a neuter definite article, Dutch uses 'het' for its definite article. English now uses 'the' without any gender but this wasn't always the case.
De = definite article for masc. and fem. nouns, exactly the same as English 'the'. I hope that answers your questions.