Copyright, Fair Use, and DMCA
A great resource for copyright and fair use information is the Stanford University Libraries Copyright and Fair Use Overview. Unfortunately, copyright infringement is rampant on the web, and measures like cease and desist letters and DMCA notices may have to be used. For example, Google will not do anything about copyright infringing pages in their index without a DMCA notice.
I've managed to reduce (but not eliminate) the unauthorized use of my content by switching my site feeds from full feeds to less tempting partial feeds and by adding image hotlink protection to my .htaccess file.