When we talk about the difference between ‘as’ and ‘like’, we mean that we compare these words as prepositions or conjunctions.
It is not so difficult to remember these meanings:
like + noun = similar to
as + noun = in the role of
Like your doctor, I'll recommend you to stay in bed also. (I'm not your doctor but I will recommend you to stay in bed in a similar way to the doctor).
As your doctor, I'll recommend you to stay in bed. (I am your doctor).
We also use ‘like’ to compare two things, appearance, behaviour. For example:
My brother bought a car like me.
This dress looks like a bathrobe.
We usually use ‘as’ for jobs. For example:
My sister works as a nurse in the hospital.
There is the only case when we use ‘like’ and ‘as’ interchangeably:
Nobody knows John like/as I do.