What is difference between unix and linux?
To begin with - marketing.
"GNU's not UNIX" used to be a big deal when the license for a real, name-brand UNIX cost $100,000 and up. These days, UNIX is a specification, not a brand, and GNU is in no risk of being suppressed for being too similar (despite SCO's best efforts). Any UNIX-like can potentially be certified as UNIX now if it's good enough and the time and effort are spent to go through the process.
Linux still hasn't undergone this certification, partly because it refuses to restrict itself to UNIX-like features. The kernel keeps getting rewritten and extended with fancy features UNIX never had, and now the foundations are changing too (how the kernel is communicated with, etc).
This is a FrequentlyAskedQuestion. I suggest to google for "linux vs unix"
They have many things in common, from a user perspective.
For instance most basic commands like cat or ls will function pretty much the same; and tools like sed and awk will exist for both.
From a systems management side, they are very different - hardware specs, layout, etc..
There are books written about this, and many articles online. What specifically are you asking about?