Periódico vs. diario
The words "periódico" and "diario" are Spanish terms that both refer to what English speakers regard as a newspaper. However, they have subtly different contexts and uses in the Spanish language.
Periódico
A1This is the general term for newspaper in Spanish. It could be published on various frequencies, not necessarily daily.
¿Has leído el periódico hoy?
(Have you read the newspaper today?)
Compra un periódico en el quiosco.
(Buy a newspaper at the kiosk.)
El último número del periódico incluye una interesante entrevista.
(The latest issue of the newspaper includes an interesting interview.)
Diario
A2Diario refers specifically to a daily newspaper or journal. It's derived from día, meaning day, implying its daily issuance.
Leo el diario cada mañana.
(I read the daily every morning.)
El diario local informó sobre el evento.
(The local daily reported on the event.)
"El País" es un diario español muy conocido.
(El País is a well-known Spanish daily.)
Summary
Periódico and diario both mean newspaper, but whilst periódico is a generic term used for any newspaper regardless of its publication frequency, diario specifically refers to a daily paper. Therefore, all diarios could be called periódicos, but not all periódicos are necessarily diarios.