Short ends are the unused part of a roll of film that is left in the magazine. Re-cans are film that was purchased for a production and loaded into the magazines, but not used. Short ends are usually about 100 to 300 feet, on average. Re-cans are full rolls.
In both cases this film is repackaged back in the bags and cans to be resold. Often it is "snip tested"... that is, the first and last foot of the roll are cut off and processed to make sure there has been no fogging, etc.
Great deals can be had, and I have purchased short ends and re-cans many times, and have never had a bit of trouble. The bargains can be substantial... my best score was several years ago when I purchased all the short ends and re-cans from the second season of "The Sopranos." I bought 50,000 feet of various stocks for less than a nickle a foot (compared to about 60 cents a foot or more from Mother Kodak). That's a lot of film for me, as I am very stingy when I shoot 35mm... and it actually took a couple of years to finally empty the freezer.
There are lots of film brokers out there, and several companies that specialze in short ends and recans. Dr Rawstock and Stock Options are two companies that come to mind... but do a quick Google search and you'll see there are tons of places.
Fantastic Plastic Entertainment, Inc. fantasticplastic.com