Porta vs. portone
In Italian, both porta and portone translate to door, but they are not interchangeable. The difference lies primarily in the size, function, and grandeur of the door in question.
Porta
A1This is the general, standard word for a door. You can use porta for almost any door: the internal doors inside a house, a regular front door, a car door, or even a refrigerator door. It is a feminine noun (la porta).
Per favore, puoi chiudere la porta della cucina?
(Please, can you close the kitchen door?)
Ho sentito qualcuno bussare alla porta di casa.
(I heard someone knocking on the front door.)
La porta posteriore della macchina è bloccata.
(The rear door of the car is stuck.)
Assicurati che la porta del frigorifero sia ben chiusa.
(Make sure the refrigerator door is closed properly.)
Questa opportunità di lavoro mi ha aperto una nuova porta.
(This job opportunity has opened a new door for me.)
Portone
A2This word refers to a large, heavy, main entrance door, typically for an entire building like an apartment block (palazzo), a villa, a church, or a historic mansion. The -one suffix is an augmentative, meaning big. It is a masculine noun (il portone).
Il portone del nostro condominio è di legno massiccio.
(The main door of our condominium is made of solid wood.)
Devi citofonare per farti aprire il grande portone verde.
(You have to use the intercom to have the large green main door opened for you.)
I turisti ammiravano l'antico portone del castello.
(The tourists were admiring the ancient main door of the castle.)
Alle undici di sera chiudono il portone del palazzo.
(At eleven at night, they lock the building's main entrance door.)
Una volta superato il portone, ho salito le scale fino alla porta del mio appartamento.
(Once I got past the main entrance door, I went up the stairs to the door of my apartment.)
Summary
In essence, use porta for any regular door you encounter inside a home, on a car, or on furniture. Use portone specifically for the large, often imposing, main entrance door of an entire building. Think of a porta as a standard door and a portone as a grand entrance or gate-like door.