French articles are tiny words used to introduce nouns.
Believe it or not, they're generally considered adjectives, and as such they match in gender and number the noun they qualify.
There are four kinds of articles in French:
le (masculine), la (feminine), l'(masculine or feminine), les (either) = the.
Note also that l' is just the elision of le or la.
These articles indicate that the noun they describe is known with certainty.
Le chat (the cat)
La tarte (the pie)
L'autruche (the ostrich)
Les avions (the planes)
un (masculine), une (feminine) = a des (either) = some
These articles in French indicate that the noun they describe is not known with certainty.
un fromage (a cheese)
une aiguille (a needle)
des moutons (some sheep)
du (masculine), de la (feminine), de l' (elision of the two others) = some
These articles in French are used when the quantity they describe can not be divided into parts (sand, water, flour).
du sable (some sand)
de la paille (some hay)
de l'eau (some water)
Don't mistake the constituent for the category.
While flour the substance can not be divided (you wouldn't say one flour, two flour without qualification), flour as a category can reasonably be thought of as being definite and singular. In French, it most definitely is.
Contrast: du caramel / some caramel AND le caramel / the caramel category
ce, cet (masculine), cette (feminine) = this ces (plural) = these
Note that cet replaces ce when the noun starts with a vowel (only used in this context)
Demonstrative articles in French are used to point emphatically to the specific object/person/animal/concept that the noun represents.
Ce politicien (this politician)
Cet avion (this plane)
Cette voiture (this car)
Ces routes (these roads)
For even more emphasis the following forms are used:
ce…ci, cet…ci, cette…ci = this…here ces…ci = these…here ce…là, cet…là, cette…là = this…there ces…là = these…there
Ces gens ci (these people here)
Cet avion là (this plane there)
Ces moutons là (these sheep there)