The correct usage of 大约 vs. 差不多
大约 means "approximately", 差不多 means "almost".

大约 is normally used as an adjective or adverb in phrases like:

大约四英尺厚的雪
snow that is approximately 4 feet thick


差不多 can be used in a similar way, but it's more commonly used as an adjective-verb:

这两本书的内容都差不多
these two books' contents are about the same


However, 这两本书的内容都差不多一样 (with 差不多 as an adverb) means the same thing. 

The important thing to remember is that 差不多 means almost, but not quite, so two things that are 差不多一样 are not exactly the same. Whereas something like 大约一个小时 could be exactly one hour, although it could also be a bit more or less.  

差不多 is also commonly used in a colloquial sense:

差不多了吧
that should do it


In my experience, 大约 is more common in written language. The spoken equivalent is 大概. But that may be a regional thing. 差不多 is very common in colloquial language, mostly in the sense of "about the same" or "good enough" as in the examples above.