what do "compositor" and "particle" mean?

What's the derivation of these words in relation to schema?  I don't
really understand the definition nor distinction between the two.  At
the moment, I'm referencing

http://www.xml.com/pub/a/2000/11/29/schemas/part1.html?page=4

for the most part as a tutorial.

compositors:
sequence
choice
all

particles:
element
sequence
choice

but would welcome pointers to better material explaining this
particular sub-topic in more detail as I haven't found a better source
as of yet.


thanks,

Thufir