Espalda vs. lomo
The Spanish words espalda and lomo both translate to the English word "back", but they are used in completely different contexts. The main difference lies in whether you are talking about the anatomy of a human being, the anatomy of an animal, or certain inanimate objects.
Espalda
The word espalda refers specifically to the anatomical back of a human being, spanning from the neck and shoulders down to the waist. It is also used to describe the back part of clothing that covers a person's back.
Me duele la espalda por estar sentado todo el día frente a la computadora.
(My back hurts from sitting all day in front of the computer.)
Ella tiene un tatuaje de una flor muy grande en la espalda.
(She has a very large flower tattoo on her back.)
El nadador tiene la espalda ancha debido a sus años de entrenamiento.
(The swimmer has a broad back due to his years of training.)
Asegúrate de mantener la espalda recta al levantar cajas pesadas.
(Make sure to keep your back straight when lifting heavy boxes.)
El vestido de novia tiene una fila de perlas en la espalda.
(The wedding dress has a row of pearls on the back.)
Cargó a su hijo pequeño en la espalda durante toda la caminata.
(He carried his young son on his back during the entire hike.)
El hombre fue atacado por la espalda y no pudo ver a su agresor.
(The man was attacked from the back and could not see his aggressor.)
Lomo
The word lomo is used to refer to the back of an animal, especially a four-legged one. Beyond animal anatomy, lomo is also used to describe the spine of a book, the blunt back edge of a knife, or a specific cut of meat like pork or beef loin.
El gato asustado arqueó el lomo cuando vio al perro entrar al cuarto.
(The scared cat arched its back when it saw the dog enter the room.)
El jinete acarició el lomo del caballo para calmarlo después de la carrera.
(The rider stroked the horse's back to calm it down after the race.)
El título de la enciclopedia está impreso en el lomo con letras doradas.
(The title of the encyclopedia is printed on the spine with gold letters.)
En el siglo pasado, la mercancía se transportaba a lomo de mula por las montañas.
(In the last century, merchandise was transported on the back of a mule through the mountains.)
Al perro viejo de mi abuelo le encanta que le rasquen el lomo.
(My grandfather's old dog loves getting its back scratched.)
El cocinero preparó un lomo de cerdo al horno con papas y romero.
(The chef prepared a baked pork loin with potatoes and rosemary.)
Rompió la cáscara del ajo golpeándola suavemente con el lomo del cuchillo.
(He broke the garlic skin by hitting it gently with the back of the knife.)
Summary
In summary, you should use espalda when talking about the back of a human being, and use lomo when referring to the back of an animal. Furthermore, lomo has a few unique usages for inanimate objects, such as the spine of a book, the dull edge of a blade, or a cut of meat.