Actually they are a little different:
1) Poco = Few, little (before an uncountable noun)
2) Pocos = Few, little (before a countable noun)
3) Un poco = A little (before an adjective)
For example:
1) Tengo poco espacio en mi habitación = I have little space in my room
2) Tengo pocos amigos = I have few friends
3) La manzana está un poco verde = The apple is a little green
About the last one ("un poco de"), you can use it in the next situations:
1) With feelings (here can be translated as "a little"):
- Tengo un poco de miedo = I'm a little scared
- Tengo un poco de vergüenza = I'm a little shy
- Tengo un poco de sueño = I'm a little sleepy
- Tengo un poco de hambre = I'm a little hungry
- Hace un poco de frío = It's a little cold
I you use "poco" in the previous sentences, the meaning changes:
- Tengo poco sueño = I'm not very sleepy (so, I don't want to sleep yet)
- Tengo poca hambre = I'm not very hungry (so, I don't want to eat yet)
- Hace poco frío = Isn't very cold (just a little, almost nothing)
2) When you offer or ask something (here can be translated as "some"):
- ¿Quieres un poco de té? = Do you want some coffe?
- Quiero un poco de té = I want some tea
- ¿Quieres un poco de pizza? = Do you want some pizza?
- Dame un poco de pizza = Give me some pizza
If you say "poco" or "pocos" here, the meaning changes, but we don't say this usually:
- ¿Quieres poco té? = Do you want few tea (or much)?
- ¿Quieres poca pizza? = Do you want few pizza (or much)?
- Dame poca pizza = Give me few pizza (not much).