orcishgamer: Wtf, how can you plagiarize yourself? This sounds like some made up, BS term, to keep students from recycling work. Because, you know, there are actual dictionary definitions of plagiarism and every one of them has this essential element in them: "another author". You answered your own question anyway. You're talking about a different element of the same subject. That's really the end of it.
Ye Olde Favorite Wikipedia on self-plagiarism. However it is actually a problem, though it can be argued to be a mostly ethical problem (but is not plagiarism in general an ethical question?). There have been cases where the same article have been published in different journals without recognition, and where researchers have just recycled same material with new headings - both of these are big no no 's.
When it comes to student work, there are three reasons to cite your own work:
1 - Ethical, is it just not right to be open with where you have the information from? even though the source is yourself? Also all "facts" or "information" in academia needs to be evidenced, in this case the evidence comes from your own previous work and it therefore needs to be cited so it can be check by fellow academics.
2- Habitual, just get into the habit to alway cite whenever you are using a source, no matter which source it is. This is a good habit that might save your own back someday.
3- Technical, many unis today are checking student work electronically. A computer will not now that the essay you handed in last year is actually your own, it only check to see how this new piece of work compares with previous submitted work. However have you cited yourself it flags out as a citation straight away.