Ciento vs. cien
The Spanish words 'ciento' and 'cien' both translate as 'hundred' in English. However, they are used differently in the Spanish language depending on the context.
Ciento
This term is typically used in cases where 'hundred' occurs before any other number, to create constructs such as one hundred and one (101), one hundred and two (102), and so on.
Tengo ciento veinte libros en mi biblioteca.
(I have one hundred and twenty books in my library.)
Me quedan ciento cinco páginas para terminar este libro.
(I have one hundred and five pages left to finish this book.)
Cien
This word is exclusively used when speaking about the number exactly equaling one hundred (100). It also occurs in idiomatic expressions.
Tengo cien manzanas.
(I have a hundred apples.)
Dio el cien por ciento en el partido de hoy.
(He gave 100 percent in today's game.)
Summary
'Ciento' and 'cien' both refer to the number 'hundred' but are used differently in context. 'Ciento' is used whenever a hundred is combined with additional numbers whereas ‘cien’ typically denotes exactly a hundred or appears in idiomatic expressions.