There are many types of plagiarism, with varying degrees of seriousness. The Imperial College of London (as part of its Olivia Environment Student project) has defined these different categories as:

Collusion
This is when one student produces work and allows another student to copy it. If both students submit the work, BOTH students will be deemed to have colluded. Collusion falls under Academic Misconduct and can result in the piece of work being failed, a percentage reduction in the overall course mark OR an overall failure of the course.

Collusion differs to group work - some coursework assessments will involve students working together on a particular project. Such assessments may require students sharing ideas, research and having a joint responsibility for the development of a project. Assignments for group work, however, should be written independently - identical assessments will be considered to be collusion.

Advice to academic staff - try to make the remit of group work assessments as clear as possible. Ensure that students understand what is expected of them, and remind them of the implications of collusion.

Advice to students - if you are unclear about the criteria for assessment of group work assignments please ask your tutor/lecturer/course convenor for clarification.

Complete plagiarism
This is when a piece of work is copied entirely from one or more sources. Even if the source(s) is acknowledged, and even properly referenced, it is still considered to be plagiarism as it contains no original work, or interpretation of the information, from the student.

Partial plagiarism
This involves inserting sections of directly copied and unacknowledged source(s) within an assignment.

Copy and paste
The availability of information on the internet & electronic journals means that it is now easy to 'copy and paste' information into assignments. It is important that if such information is included in assignments that it is included in quotation marks ("....") and that it is properly referenced.

Word switch
If you copy a sentence or paragraph into your assignment and change a few words it will still be considered to be plagiarism. It is better to paraphrase than to quote, and if you copy a phrase you should copy it word for word and use quotation marks.

Misinterpreting common knowledge
Common knowledge is information which is (a) well known to all in a particular field, (b) easily verified by consulting standard textbooks or encyclopaedias, and not disputed, (c) undisputed historical facts, and (d) known formulas or equations.

Concealing sources
If you have cited a piece of work from a text this does not mean you do not have to reference any other text you refer to from that work. No matter how many times you refer back to the text you must acknowledge the source, even if it is in the very next paragraph.

Self-plagiarism
Self-plagiarism is when you re-use your own previously written work or data in a new assignment and do not reference it appropriately. If you use material from a previous assignment you must reference it appropriately. Never use the same essay for different lecturers. If re-sitting a course, do not submit the same essay.

Inadvertent plagiarism
Plagiarism can occur as a result of poor study skills, but it is still considered to be academic misconduct.

Real examples of plagiarised & non-plagiarised work
The University of Teeside has produced an online tutorial that explains these different types of plagiarism using real examples of work. Each example is accompanied with an explanation as to why a piece of work is considered to be plagiarised (or not!). It is really useful to have a look at this tutorial to ensure that you fully understand what is and is not plagiarism.

Further link
The 'OWL at Purdue' (Purdue University, America) has produced a clear and brief overview of what is and isn't acceptable practice.