vs
QUICK ANSWER
"Los detalles" is a form of "detalle", a noun which is often translated as "detail". "Fijarse" is a pronominal verb which is often translated as "to pay attention". Learn more about the difference between "los detalles" and "fijarse" below.
el detalle(
deh
-
tah
-
yeh
)A masculine noun is used with masculine articles and adjectives (e.g., el hombre guapo, el sol amarillo).
1. (fact)
a. detail 
Me gustaría saber todos los detalles de lo que sucedió.I would like to know all the details of what happened.
2. (present)
a. little gift 
Les compré a todas las compañeras del trabajo un detalle en Jamaica.I bought a little gift in Jamaica for all my coworkers.
b. little something 
Gracias, Marcela. Eres muy amable. - De nada; es solo un detalle.Thanks, Marcela. That's very kind of you. - You're welcome! It's just a little something.
3. (courtesy)
a. nice gesture 
El ramo de flores fue un detalle muy bonito.The bouquet of flowers was a very nice gesture.
4. (decoration)
5. (economics)
a. statement 
Si desea ver el detalle de su factura, puede hacerlo en la página web de la empresa.If you wish to see your bill statement, you can do it through our company's web page.
fijarse(
fee
-
hahr
-
seh
)A pronominal verb always uses a reflexive pronoun. (e.g., Te ves cansado.).
a. to pay attention 
Fíjate bien en todo lo que dice y luego me lo cuentas.Pay close attention to everything he says and then tell me afterwards.
a. to notice 
No me había fijado en el escalón y casi me caigo.I hadn't noticed the step and almost fell.
fijar
A transitive verb is a verb that requires a direct object (e.g., I bought a book.).
3. (to put)
a. to fix 
Tienes que fijar la lámpara al techo con los tornillos.You have to fix the lamp to the ceiling using the screws.
b. to establish 
Ha fijado su residencia en Londres por cuestiones de trabajo.He has established his residence in London because of his job.
c. to fix 
Antes de decidirnos por una estrategia, debemos fijar nuestros objetivos.We need to fix our objectives before deciding on any strategy.