Just back from Berlin & the Musikfest...wow! - so have just been informed of this thread (thanks, Jen! x)
A canon is simply a melody which is played or sung by a group of performers. A theme/melody begins with a portion of the group performing it. After a while another section of the musicians begins the same tune whilst the original performers are continuing on their way. A third or fourth entry may or may not occur, depending on the construction of the melody.
On reaching the end of the piece, if the first performers begin again from the beginning, and all other sections follow suit as they in turn reach their ends, then this particular type of canon is called a
round as it can go round and round for as long as the performers wish.
Although a
fugue contains canonic writing, the form is far too complex to go into here, and a fair degree of advanced theoretical knowledge is essential before attempting to analyse the various constructional devices which may be employed. To master the art of constructing a successful fugue is one of the highest forms of compositional attainments, and anyone able to bring this off may justifiably be proud of him/herself.
This is typical of the way in which fugal composition is outlined.
Here again is another site (if unfamiliar with serious academic musical study it can make your head hurt, can it not?) I have tried to find the simplest websites (!) If any of you can find a more 'user friendly' site, please post it here.
Cheers!

Ritz.
PS The Pachelbel Canon is, in fact,
not a canon (!) There are three sets of strings which perform canonically above a
Ground Bass which would be the correct title, e.g. see
'Purcell's Ground'.