Using "¿qué? and ¿cuál? in interrogative sentences; You know them better as "questions".

Both ¿qué? and ¿cuál? mean "what?" in English, but the verbs in Spanish change depending on where they are used.  "cuál" comes before the verb ser for the most part, or it comes before a posted selection.  If before a posted selection, it takes the meaning of the English "which?".  

¿Cuál fue la mejor parte de la película? What was the best part of the movie?

¿Cuál es el día de hoy? What is today's date?, What's today?, or What day is it?

¿Cuál vestido quieres usar, el azul o el blanco? What (which) dress do you want to wear, the blue or white one?

When we want to refer to several objects, we use ¿cuales?.  Since the penultimate syllable is naturally accented here, we drop the diacritical mark, and make ser the third person plural conjugation:

¿Cuales son los restaurantes que a ti te gustan más? What are the restaurants that you enjoy the most? 

Qué is used for only specific grammar utterances when meaning what? We use qué when telling time, saying the date, referring to a specific noun, or asking the general definition of something.  Observe, por favor: 

¿Qué hora es?  What time is it?

¿Qué mochila es la tuya?  What backpack is yours?

¿Qué quieres hacer hoy? What do you want to do today?

¿Qué es Mazatlán? ¿Es el nombre de una ciudad?  What's Mazatlan? Is that the name of a city? 

When you're astounded by something, and you're like "What the heck is that?", you simply use the neuter form of a demonstrative adjective, eso or esos.  Notice that this is emphatically done before the verb ser, unlike previously.  

¿Qué es eso?  What is that? 

¿Qué son esos? What are those?  

Ojalá que te ayude, 

Profesor Joel