What is the difference between 'le, la, les' and 'un, une, des'?

'le, la , les' are called definite articles or articles définis and they are used when speaking of something specific or that has previously been mentioned. These articles are the equivalent of 'the'.'un, une, des' are called indefinite articles or articles indéfinis and they are used when speaking of something unspecific. These articles are the equivalent of 'a'.
For both types of articles, definite and indefinite, the article has to agree with the gender of the noun it precedes. This only happens when the noun is singular. If the noun is plural, the article will not need to agree with the gender of the noun. Instead, the plural article will be used ('les' or 'des').