Accomplishments are things you started, completed, worked on,
created, developed or made possible--things that happened
because you were there. It can be a long or short term project,
something created or supervised with others, or by yourself. But
they're always specific, not general, and they are always things
in which you played an active role, even if others worked with
you. Look at the difference between a duty or responsibility
(which does not market you effectively but is the way most
people write their resumes), and the same situation described as
an accomplishment.
Here's a duty/responsibility that like those that appear on a typical resume:
"Wrote weekly reports on sales and submitted these to home office."
Now as an accomplishment the same information in selective detail:
"Completed 134 summary reports on sales, including weekly volume, percent of increase, and
new clients seen. Received commendation from sales manager for accuracy and for never
missing a deadline."
Note that the accomplishment described the duty using "quantifying and qualifying" words.