Copyright law works on the Internet and the Web just like it does everywhere else: If it's not yours, you have to assume someone else created it, owns it, and deserves to be paid for it before you can take it or use it.
Unless the software or graphics or text says it's free, assume that it is copyrighted. This means you can copy for personal or educational use, but you CANNOT reproduce it, publish it, or say that it is yours.
It is better to err on the side of safety in this area since some companies are getting particularly aggressive at pursuing and jailing people who break copyright laws.
If you have more detailed questions, do a search on the Web and do some reading. There is LOTS of information out there on the law.