Copyright regulates the re-use of other people's work.
To re-use copyright-protected material you must obtain the permission of the copyright holder. It can be that a re-use (or licensing) arrangement is specified with the publication (e.g. this photo may be used for educational purposes), but if this is not given explicitely, permission must be obtained.
Copyright covers for example figures, diagrams, photos, tables, sound, video, text...
The copyright for a work lies initially with the creator.
Copyright comprises two parts: an economic part (material rights), which can (and often are) signed away to a third party (often a publisher), if the work is published, and a moral part, which cannot be signed away. The moral part requires that anyone re-using someone else's work must give credit to the creator.
Copyright on a work exists during the lifetime of the creator and for 70 years after his/her death.
Proper referencing protects you from plagiarizing, but it does not give you the permission to re-use copyright-protected material. The main exception to the latter is quotations of small amounts (say up to a couple of hundred words) of text, however a reference must be given.
Material available on the internet is also copyright protected. Implicitely your browser may download it for you to read; you may not use it beyond that without permission.
Because of the organization Bonus Copyright Access in Sweden, teachers in Sweden can make use of copyright protected material for educational purposes. This material can be distributed by
a) save digitally on the institution's closed network,
b) use the copied material in presentations such as PowerPoint,
c) store and distribute digital presentations on the university´s closed networks, email, d) show on the screen and e) handed out to registered students in paper or electronic copy.
Small amounts of a work may be copied for private use (there is no precise definition of "small", but as a guideline you can use 15 pages or 15%, whichever is less). Private use includes a student's individual studying, but does not cover including material in assignments.
Once you have permission to re-use someone else's work, remember to reference the source.
If you cannot find the copyright holder or cannot contact them, you may not re-use the material.
However, if a student's work will be submitted only to obtain examination and not posted publicly i.e. distributed in print and / or electronically via the Internet, no permission from the copyright holder is needed to use photos, graphs, figures, etc. If the work instead is to be distributed in print and / or electronically via the Internet, permission is required to use photos, graphs, figures, etc.