Answer: le is an article while de is a preposition. An article is used in order to qualify the nature of the noun (determined = the, undetermined = a, etc.). A preposition is used in order to qualify the nature of the relationship between a subject and an object through the action of the verb. Thus: Il (subject) vient (verb) de (from) Paris.
Confusion arises often in French due to the use of the determined article inside of a preposition such as ‘a‘ or ‘de‘. It is possible to say ‘je m’en vais au village’ (I go to the village), where the preposition a is fused with the ending u, that substitutes the article. The same for ‘de’, where in the sentence ‘Je viens du village’, the article is again fused with the preposition. This is only the case for the male article ‘le‘, whereas they are kept separated while using female articles ‘Je m’en vais a la ville’ or ‘Je viens de la ville’.
For all other prepositions even with male articles, they’re kept separated. ‘Il fut poussé contre le mur’ (He was pushed against le wall), ‘La poire est sur l’armoire’ (The pear is on the closet.) Etc. Endings of articles disappear when the noun following starts with a vowel, as well for male as for female articles.