There are two grammatical genders in Danish: common and neuter. All nouns are mostly arbitrarily divided into these two classes. The singular indefinite article (a/an in English) is en for common nouns and et for neuter nouns. They are often informally called n-words and t-words.
En dreng. A boy.
Et fængsel. A jail.
All from wikipedia, if you want to look into it more: