Ordinarily, it's something you've purchased from an authorized source. On occasion, lawful copies may be acquired via Interlibrary Loan, or when the copyright owner chooses to give something away. In all cases, you do NOT have the right to systematically redistribute your lawful copy whether free or not, though you may have a First Sale right to dispose of your copy.
First sale rights extend only to the physical copy you possess, and only for certain media: books, CDs, DVDs, video. By law, you're allowed to resell the physical item as long as you don't retain a copy for yourself. A distinction not always recognized is that ownership of the physical item, such as a book or a CD, is not the same as owning the copyright to the work embodied in that item. Under the first sale doctrine, ownership of a physical copy of a copyrighted work, like a book, permits lending the item, reselling the item, disposing of the item, and so forth, but it does not permit copying the item in its entirety. That is because the transfer of the physical copy does not include transfer of the copyright to the work.
Yes. Licenses provide restrictions on the ownership and use of materials above and beyond copyright law. Most software is now protected by licenses to which you must agree before you can finish the installation. Be aware that the licenses for most software you purchase supersede first sale rights and do not allow you to resell your physical copy of the software. Online journals are routinely protected by licenses that specify how and to whom we can make the content available.
It means that you may copy protected works for personal reasons, whether work-related or not. You may copy only a small part of the whole, and may not share the copies with others. For example, you may have fair use rights to copy articles that support your research or educational activities. You may have fair use rights to create a backup copy of purchased software. You do NOT have a fair use right to borrow a book and make an entire copy of it. You may not make multiple copies for friends or colleagues--unless it's for classroom use.
You can't change it enough to matter. Copyright law allows only the copyright owner to make adaptations of original works.
If it's in print, you may have a fair use right for one use in a classroom setting. Subsequent uses would require permission of the copyright owner. There are restrictions on the number of images that can be taken from one source--10% or 15 images, whichever is less.
Comic strips are ordinarily copyrighted, and these rights are strongly enforced by the syndicates. Our advice is to resist using cartoons in any digital presentation without permission of the copyright owner. There are no fair use arguments for scanning comic strips.
In a classroom setting, you may play a lawfully acquired song, but not the entire CD. You may make a portion of this song available to your distance students--but the portion is less than that for face-face instruction ("reasonable and limited"). The TEACH Act requires that your use of any multimedia be an integral part of what you teach.