It seems that there is no place on the internet that properly explains the AHL PTO, so I will do my best to fill that void. Please let me know if there are any errors.
PTO = Professional try-out contract
This is an animal found only in the AHL. PTO's are for 25 games, but the player can be released before that. He can also be signed to a second PTO, but after two PTO's he either becomes a free agent or is signed to a SPC - standard players' contract (also just called an "AHL contract").
Another type of contract in the AHL: ATO = Amateur try-out contract
These are signed by previously amateur players -- graduating college seniors and players finishing juniors. There is no set number of games on these.
Any player on an AHL SPC, PTO or ATO must be released from his contract before he can sign in the NHL. Occasionally you'll hear people say an NHL team "picked up his contract," but he's really signing a different kind of contract, it's not directly transferable. The player is not eligible to play in the NHL on a PTO or even a SPC. To play in the NHL you must be signed to an NHL contract, which are full-year, guaranteed contracts (with exceptions for goaltenders in emergencies and players sent back to junior. There is also a one-game ATO in the NHL which is extremely rare, but used for players coming out of college sometimes. See Justin Abdelkader of Detroit).
So a way to think about it is that the PTO is sort of an intermediate step between the rigidity of NHL or standard AHL contracts, and the complete job insecurity of ECHL contracts, which can be terminated at any time (though the player does first go through waivers).
On a related topic, NHL teams cannot "sign players to an AHL contract." This wording is often used by teams who own their AHL affiliate (like Edmonton), but really what is happening is that they are directing the AHL team to sign the player to an AHL contract. It just sounds better to say they did it. Unless you see an official transaction that says the NHL team signed the player, be skeptical.
NHL teams sign players to NHL contracts, of which they are limited to just 50. AHL teams can sign as many players as they like, since there is no roster limit or salary cap in the AHL. Since there is no roster limit, there is also no formal injured reserve (IR) in the AHL. Teams don't give out much information about who is injured, because they don't have to.