An aphorism is a brief statement of truth/opinion/belief, at its longest still too short to be a full-fledged paragraph.
I do not think of aphorisms the same as quotes, common expressions, or maxims. These may impart some reasonable insight, but often do so in a superficial manner, sometimes employing simple linguistic patterns or tricks in the place of actual substance.
Aphorisms do not appeal to the least common denominator. An aphorism won't utilize a simple and uninteresting metaphor like "if you can't stand the heat..." just to be easily accessible. An aphorism earns its place among philosophical literature by illustrating how much wisdom can actually be extracted from so few words.
Feel free to post your own aphorisms, or those of another sagely thinker. If you post someone else's thought, please credit him/her (duh). If it is your own work, you don't need to explicitly credit yourself.
In keeping with the very purpose of the community, keep your post short. I am hesitant to mandate a particular numerical limit on number of lines or words, so just go by the aforementioned criterion: if it starts looking like a paragraph, you've written too much.
Other than that, be as unfettered, revolutionary and taboo as you please. Just don't practice disrespect for its own sake.