Conditioner vs. Serum: Which Hair Treatment is Best?

Question
I wanted to know which is better to be used after shampoo: conditioner or serum?
and what's the difference between them?
Dana i wanted to tell you about something else, i've used castor oil for taller and thicker eyebrows and actually it worked really good :)
my eyebrows are a little thin (i mean they almost donot appear in pictures!) but their length is nice no problem with it,,
i considered putting some of the castor oil on them but thought that they might get more thick and long ,,
but i'hv not tried it lest they become more long in a way that makes them look not that good,,
what shall i do?
is it okay to use castor oil (with or without olive oil) for thicker eyebrows?
or there is another better way?
thanks :) :)

Answer
As far as using the castor oil for your brows,  first you said you tried and it worked, then you said you haven't tried it because you're worried they'll grow out badly. I'm not sure i understand the question. As fir me personally, I'd rather use brow powder than put castor oil on my face.

Conditioner is designed to treat and repair specific issues you're having with your hair. Serum, while some of them do repair, are more designed for maintenance, adding moisture and shine. Shampoo, then condition, then serum.