Both words have many uses, but what mainly causes problems for Spanish speakers when they both translations of 'como', particularly when they're followed by a noun - for example, the difference between 'like a tourist' and 'as a tourist'. It will help you understand the difference if you remember that 'like' means 'similar to'. For example: A crocodile is like an alligator. A clementine is like an orange. These two animals and fruits are not the same, but they are similar, so we use 'like' - it's a comparison. 'As' means 'in the role of' or 'in the capacity of'. For example: He works as an architect This means that he really IS an architect - that is his job. So if you compare these two sentences - They worked as slaves. They worked like slaves. - the difference should be clear. In the first, the people really were slaves - this might be a line from a history book. In the second, this is just an idiomatic use, a figure of speech or an exaggeration, just meaning that they worked very hard. If you are walking round your own city with a friend and he is taking lots of photos, you could say that he is 'like a tourist'. He isn't really a tourist - he is just similar to one. However, you might say that someone from your country went to the USA 'as a tourist' - this means that this was his status on his visa i.e. the capacity in which he visited the country. Or in a language context, you might begin a sentence 'As a native speaker,....' meaning that you really are a native speaker of that language. Or if you aren't a native speaker of that language but you speak it so well that people imagine that it's your mother tongue, you might say 'I speak it like a native speaker'. I hope that helps.

These words have many usages and there are a number of rules that you would need to learn. Have a look at this : http://www.grammarly.com/handbook/grammar/adjectives-and-adverbs/26/misuse-of-like-and-as/ There are plenty of similar explanations on grammar websites. You could produce your own sentences and ask for them to be corrected, if you like. Then we can see if you have understood well the various usages.

In many cases they are interchangeable. I would be helpful to see a specific example if you have one in mind.