Full Idea
A collection has a determinate number of members, whereas a fusion may be carved up into parts in various equally valid (although perhaps not equally interesting) ways.
Gist of Idea
Collections have fixed members, but fusions can be carved in innumerable ways
Source
Michael Potter (Set Theory and Its Philosophy [2004], 02.1)
Book Reference
Potter,Michael: 'Set Theory and Its Philosophy' [OUP 2004], p.22
A Reaction
This seems to sum up both the attraction and the weakness of mereology. If you doubt the natural identity of so-called 'objects', then maybe classical mereology is the way to go.