I have always understood things as Irma said.
To hear = oír = the physical sense of detecting a sound
To listen to = escuchar = make sense of these sounds
However, my dictionary indicates there is quite an overlap, both in English and in Spanish:
oír = to hear
no oiga nada
I can't hear anything
he oído hablar de él
I've heard of him
oír = to listen to
oigo la radio por la mañana
I listen to the radio in the morning
el juez oyó a los dos partes
The judge heard both sides
The last example is interesting, because
oír and
to hear are used in clear sense of
to listen to (hopefully)
oír misa
to go to mass (where one hopes the people might actually be
listening 
)
escuchar = to listen to
no me escuchaba
she wasn't listening to me
escuchar = to hear
habla más fuerte que no te escucho
speak up, I can hardly hear you
However, the last meaning of escuchar =
to hear is given specifically as Latin America
Language is so woolly, isn't it?
