Cien vs. ciento vs. centenar
In Spanish, the words cien, ciento, and centenar all relate to the quantity of 100, but they are not interchangeable. Each has a specific grammatical function and context in which it is used, distinguishing between an exact number, a part of a larger number, and an approximate group.
Cien
A1Cien is an adjective used for the exact number 100 when it comes directly before a noun (both masculine and feminine) or before the numbers mil (thousand) and millones (millions).
Compré cien manzanas en el mercado.
(I bought one hundred apples at the market.)
El edificio tiene cien pisos de altura.
(The building is one hundred stories high.)
Hay cien personas esperando en la fila.
(There are one hundred people waiting in line.)
Cien mil personas viven en esta ciudad.
(One hundred thousand people live in this city.)
La compañía tiene un valor de cien millones de euros.
(The company is worth one hundred million euros.)
Ciento
A2Ciento is used to form compound numbers from 101 to 199. It is also used when stating the number 100 by itself (not followed by a noun), and in the expression por ciento for percentages.
El libro tiene ciento ochenta y seis páginas.
(The book has one hundred and eighty-six pages.)
Mi abuelo vivió hasta los ciento dos años.
(My grandfather lived to be one hundred and two years old.)
¿Cuántos invitados vienen? Vienen ciento veinte.
(How many guests are coming? One hundred and twenty are coming.)
El cincuenta por ciento de los estudiantes aprobó el examen.
(Fifty percent of the students passed the exam.)
Ella contó hasta ciento antes de abrir los ojos.
(She counted to one hundred before opening her eyes.)
Centenar
B1Centenar is a masculine noun that means "a set of one hundred" or "about a hundred". It is used to talk about an approximate quantity and is followed by the preposition de when a noun comes after it. Its plural form is centenares.
Un centenar de manifestantes se reunió en la plaza.
(About a hundred protesters gathered in the square.)
El autor ha escrito un centenar de artículos.
(The author has written a hundred articles.)
Se han vendido centenares de copias de su nuevo disco.
(Hundreds of copies of his new album have been sold.)
La granja tiene un centenar de gallinas.
(The farm has a hundred chickens.)
Recibimos más de un centenar de solicitudes para el puesto.
(We received more than a hundred applications for the position.)
Summary
In summary: use cien for exactly 100 right before a noun (cien libros). Use ciento for numbers between 101 and 199 (ciento cinco), for percentages (diez por ciento), or when 100 stands alone. Use centenar as a noun to mean an approximate group of one hundred, similar to "a hundred of" (un centenar de personas).