Difference between 'trabajar para' and 'trabajar por'

Question

I know the difference between por and para, but I'm confused by these particular usages. I've seen both of them in various places. 'Trabajar para' seems to mean to work for, whereas 'trabajar por' seems to be to work on behalf of. Is there any difference between them? How should I pick which one to use?

Answer 1

"Trabajar para <alguien>" means to work for someone else:

Él es mi empleado. Trabaja para mí.
He is my employee. He works for me.

"Trabajar por <algo>" means to work to advance one's own cause or ends:

Él trabaja por sus hijos.
He works for his children's sake.

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UPDATE: As suggested by @Trufa, "trabajar por <alguien>" could mean to work in place of someone else:

Ella no puede venir hoy. Hoy él trabajará por ella.
She cannot come [to work] today. Today he will perform her duties.

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This is quite flexible, though, and some people use "trabaja para" when they should use "trabaja por":

Él trabaja para ver hechos realidad sus sueños. (incorrect, but common)
Él trabaja por ver hechos realidad sus sueños. (correct)
He works to see his dreams become true.

Answer 2

The most common uses:

You'd use "por" in case of contract type:

• trabaja por cuenta ajena; (hired work)
• trabaja por cuenta propia / trabaja por libre; (freelance)
• trabaja por horas;

You'd also use "por" in case of the work's objective:

• trabaja por la paz en el Medio Oriente;

You'd use "para" in case of employee/employer relation:

• trabaja para la empresa X;
• trabaja para su jefe;

Less common uses:

• trabaja por internet; (working remotely via Internet)
• trabaja por Juanito; (meaning: he's working as Juanito's temp replacment)