Piracy and copyright

Well, I must admit that piracy is certainly a widespread phenomenon in all forms of media. I think most people have had copies of tapes, videos, CDs and even high end computer programs that aren't paid for. It's definitely a white-collar crime. Its especially tempting in the cash-deprived education system where it has the added justification of being in the best interest of your students.
I must say that I find the issue of copyright very challenging. So much is essentially free for the taking on the web so how are you to know what is and isn't considered copyrighted? Many teachers have websites where they post their notes, assignments, worksheets etc. Are these copyrighted or are they considered free for anyone as they are easily downloaded and any student, at the very least, can have a copy? Are images and pictures ok to take? Certainly a lot of websites advertise lots of free images so on another website where the picture isn't stated to be copyright protected its easy to assume the site originally got it free themselves. What if you take someone else's posted note or worksheet but then modify it to suit your course/class, would that still be considered against copyright regulations? Many sites where the work is clearly original are also clearly intended to allow free sharing including sites like YouTube and and many photo sites where people can post their original material for everyone to share. Can those people still claim copyright if you use it elsewhere?
So many questions, so few clear answers and all those "free" resources on the web sure are difficult to pass up as a busy teacher!