Freír vs. sofreír

Both 'freír' and 'sofreír' are frequently used terms in Spanish cooking instructions or recipes, which might lead to some confusion. Despite similar processes, they do have subtle but important differences.

Freír

'Freír' is a general term that means 'to fry'. It can be used to describe the process of cooking any food in hot oil or fat. The aim is normally to achieve a crispy golden exterior.
Voy a freír unas papas para la cena.
(I am going to fry some potatoes for dinner.)
Deberías freír el pescado en aceite caliente.
(You should fry the fish in hot oil.)
Siempre me quemo cuando intento freír huevos.
(I always burn myself when I try to fry eggs.)

Sofreír

'Sofreír' is a specific form of frying where food is cooked slowly over a low heat until it becomes tender and its flavors intensify. It usually refers to the slow cooking of ingredients like onions, garlic or vegetables before adding main ingredients.
Primero debes sofreír las cebollas en la sartén.
(First you should sauté the onions in the pan.)
Para hacer una salsa sabrosa, sofría el ajo antes de añadir los tomates.
(To make a flavorful sauce, sauté the garlic before adding the tomatoes.)
"Sofreir" la carne antes de cocerla le da un sabor extra.
("Sauteing" the meat before stewing gives an extra flavor.)

Summary

'Freír' and 'sofreír', while similar, differ by the cooking intensity and method. 'Freír' denotes a faster frying process at higher temperatures, while 'sofreír' refers to slower cooking over low heat, typically as a precursor to further cooking with additional ingredients.