A voucher is the paper that you are issued when you purchase an AP either online ar the Disney website, by phone from the Disney Club or in person from your local Disney Store. It is not an actual AP and must be exchanged for one once you arrive at WDW in order to gain admission to the parks.
APs are good for one year from the day that they are activated, not from the date of purchase. Many people will purchase their APs in advance if a price increase appears to be on the horizon or if they want to spread the cost of buying admission media over a period of time.
According to a number of posts that I've read, you have an unlimited amount of time to activate your AP once it has been purrchased, so if your anticipated trip is cancelled for whatever reason, you will not lose out on the money you spent on the voucher.