"Long" is the trade direction. It can be for 10 seconds, or 10 years or more etc. Basically you expect the stock to go up in price.
"Short" is the other side, you sell the stock (by borrowing it) and hope it falls in price so that you can buy back and cover your loan.
As simple as that. For eg, I went long on KOOL today. No short positions right now, but I was Short on LEH for a small time last year, unfortunately got spooked and covered too soon, missed the ride to o.. haha