':Like' means 'similar to':
A crocodile is like an alligator.
'As' means in the role or capacity of:
He works as a waiter in a restaurant.
Compare these two sentences:
'Don't wear a Hawaiian shirt and camera in town - you look like a tourist.'
This means that you resemble a tourist. You aren't a tourist, but people might think that you are one.
'You can visit this country as a tourist for a few months.'
This means that you 'are' a tourist - it's is your official status on your visa.
We also use 'as' for comparatives - 'Spanish isn't as difficult as Russian.'