An Overview of todo, todos, toda, and todas:

The words todo, todos, toda, and todas have two main functions:

a) as an adjective meaning "all" (to describe a noun), e.g.:

• Yo leí toda la historia.

b) as a pronoun meaning "all of it/them" (to replace a noun), e.g.:

• Él se comió todo.
• Todos van a ir a la fiesta.

The form you use must agree in gender and number with the corresponding noun (i.e.: the noun being modified by the adjective form of todo/a(s) or the noun being replaced and referred back to by the pronominal form of todo/a(s)). Note also that todo/a(s) must be followed by a determiner (e.g.: el, la, etc.). This is illustrated in the following table: