Raspberry Pi is a small computer that can be embedded with other components like keyboard and monitor and can be used as a normal computer. On the other hand, Arduino is a micro controller that can be used for specific tasks. A few people don't know that both are different types of devices and end up in asking 'Which one I should prefer? Raspberry Pi or Arduino'.
An article written by Ruthe Suehle of Red Hat, compares both Arduino and Raspberry Pi and explains which one to prefer for different purposes.
I spend a lot of time at conferences and events like Maker Faires, and having co-authored a book on the Raspberry Pi, I spend a lot of time talking to people about things like small electronics and open hardware. Probably the most frequent question I hear is, "Should I get a Raspberry Pi or an Arduino?"Depending on the person asking, I've almost always said yes to getting a Raspberry Pi, and not just in hopes of selling a book or two. Often the people who ask this question are at a point where they don't even realize that the two devices aren't the same. This isn't "should I get a Mac or a PC?" or "should I try Ubuntu or Fedora?"