Copyright law regarding posting unoriginal material (how does it work?)

First off, I feel incredibly cool for having just joined this forum.

Moving on to my question: I'm interested in the legal stance on using potentially copyrighted material (say, pictures and quotes showing up in Google searches/on DeviantArt/you get the idea) on ones own website, given that you provide information about the material on the site not being one's own work, as well as clearly pointing the viewer to the original source.An example of this would be that which happens over at Tumblr a lot, both through reposts and through uploading non-original material from other sources.

What does the big book of laws say about this? I am not stationed in the US and frankly have little idea of how these things work. Does it differ between countries or is there an Ultimate Law of The Internet (ULoTH) that I should be aware of?

Finally, is there any change to the verdict if one were to make money off the site at which this (unoriginal) material is posted, for instance through advertising?

If you have knowledge, feel free to share. Do, however, please refer from simple speculation. I've got myself for that.