When do I use des and when do I use les.

[earnest ridge]

"Les" is a plural, definite article. It refers to a specific noun or in general about a group of nouns.
Ex: Les frites sont délicieuse.
"Des" is an indefinite article, as we as a partitive article. In general, it is used to mean "some."
Ex: Prenez des photos.

[civic canopy]

they're both articles for multiple instances of a noun.
the difference is that "les" is a definite article and "des" is an indefinite article.

"les" can indicate specific instances of something

• les chats de Thomas sont beaux
  Thomas' cats are beautiful (you're talking about specific cats, not just any cats)
  this use case is often translated as "the" in English.

"les" can also indicate a general concept

• j'aime les chats
  I like cats (you're talking about cats in general)
  this use case is generally no article in English.

"des" on the other hand is used to indicate any multiple instances of something

• des chats sont venus ce matin
  some cats came this morning (you're not talking about specific cats, could be any)
  this use case is usually "some" or no article in English

note that both "les" and "des" are plural articles.
The singular forms for "les" are "le" "la" "l' ".
The singular forms for "des" are "un" "une"

also, note that "les" and "des" both have other grammatical functions outside of being articles.
"je les ai vues" (object pronoun)
I saw them
"j'ai peur des chats" (de + les mandatory contraction)
I'm afraid of cats