Porche vs. véranda
While both porche and véranda can be associated with the English concept of a porch, they refer to very different architectural structures in French. A porche is an open, covered entryway, whereas a véranda is a fully enclosed glass extension of a house.
Porche
A porche is a covered shelter projecting in front of the entrance of a building, such as a house or a church. It is exposed to the outside air and is primarily designed to protect the front door and arriving visitors from the weather.
Il a garé son vélo sous le porche pour le protéger de la pluie.
(He parked his bike under the porch to protect it from the rain.)
Le porche de cette cathédrale est orné de magnifiques sculptures en pierre.
(The porch of this cathedral is decorated with magnificent stone sculptures.)
Nous avons attendu sous le porche que quelqu'un vienne nous ouvrir.
(We waited under the porch for someone to come and open the door for us.)
Le vieux chien dort souvent sur les marches du porche.
(The old dog often sleeps on the steps of the porch.)
Ils ont installé un petit banc sous le porche de leur maison.
(They installed a small bench under the porch of their house.)
Véranda
A véranda is a closed, glass-walled room attached to the outside of a house. Unlike the English word which often implies an open, wrap-around porch, a French véranda is always enclosed with windows and serves as a bright, year-round indoor living space similar to a sunroom or conservatory.
Nous prenons souvent notre petit-déjeuner dans la véranda pour profiter de la lumière du matin.
(We often have our breakfast in the sunroom to enjoy the morning light.)
Ses parents ont fait construire une véranda pour agrandir le salon.
(His parents had a conservatory built to enlarge the living room.)
Les plantes tropicales poussent très bien dans la chaleur de la véranda.
(Tropical plants grow very well in the heat of the glass extension.)
Il pleut dehors, mais nous sommes au chaud dans la véranda avec un bon livre.
(It is raining outside, but we are warm in the veranda with a good book.)
Le chat adore faire la sieste derrière les vitres de la véranda.
(The cat loves to take a nap behind the glass windows of the sunroom.)
Summary
To summarize, you should use porche when referring to an open, roofed structure that shelters the main entrance of a building. You should use véranda when talking about a fully enclosed, glass-walled room attached to a house that functions as an extra indoor living area.