Articles are tiny words used to introduce nouns. 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:
1) Definite articles:
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
2) Indefinite articles:
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
3) Partitive articles:
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
CAREFUL! Do not 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
4) Demonstrative articles:
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