What's the main difference between 为 and 是?
为 has a lot meanings, and so does 是. A common meaning between 为 and 是 is "to be" or other variations of "to be", like "being". 为 is used more for its other meanings than "to be", while 是 is used more for "to be" than its other meanings. When meaning "to be", in some cases, 为 and 是 are interchangeable. For example: 


  
  我们今年的工作目标 为 ... 
  我们今年的工作目标 是 ...
  


But there are some cases where 是 cannot be replaced by 为, for example: 


  Yes: 我 是 一名学生.
  No: 我 为 一名学生(X). 


为 has another meaning very close to "to be", which is "regard ... as ... " in the pattern of 以...为..., for example 


  以 厂 为 家, 以 人 为 本, 民 以 食 为 天, 以 救国救民 为 己任, etc. 


In this case, 为 cannot be replaced with 是. By the way, 为 and 是 can even be used together, like 自以为是. But here 为 means "believe", and 是 means "right", or "correct". 自以为是 means "consider oneself always right". 自以为是 is a derogatory term, used to show dislike, disagreement or disapproval.

Another example: 


  是为序。


Here 是 means "this", 为 means "is", 序 is "preface", so the whole sentence means "this is the preface". It is sometimes used to end a preface.

One more example: 


  何以为是，何以为非？无以为是，无以为非。 


Here both 是 mean "true" or "truth", both 非 mean "false" or "falsehood". All the 4 为 mean "to be". 

According to above-mentioned samples, 为 means that it is NOT xxx but very like xxx. In Chinese words, (可能它一定不是xx，但也可能它现在不是xx，将来可能是, 但)把它当作是xx。

Example: 以厂为家，厂 is not home，but someone treats it as home。 


  今年的目标为xx，i.e. 我们把xx当作今年的目标。 


为 implies that we assign a role to a thing actively/explicitly, but 是 implies that a thing does natively have the role.