The Changeling is a temporary Zerg unit spawned by an Overseer that lasts for 150 seconds. The Changeling spawns as a Zerg-looking unit, but upon seeing an enemy unit or building it will automatically transform into the basic unit of that enemy's race (Marine, Zergling, or Zealot). If the enemy player has researched Combat Shield for Marines or Metabolic Boost for Zerglings, then the Changeling will also copy the upgraded models for those units. The Changeling will not gain the new model if the enemy player researches one of those upgrades after the Changeling has already morphed.
Once disguised, the Changeling takes on the enemy's own color and will no longer be automatically attacked by the enemy's units, allowing it to infiltrate enemy territory unsuspected. Changelings cannot be discerned as impostors by any form of detection, nor will the secondary effects of EMP Round reveal their true allegiance. Likewise, a Sensor Tower's Radar ability will not identify the Changeling as hostile after it has morphed. However, if the opponent selects the Changeling, they will be unable to give it orders (since it is not theirs) and the word "Changeling" will appear in place of the unit type that it appears to be. In the event that an opponent does not spot a Changeling joining their army, it will often be revealed when they notice that it doesn't have a selection circle around it when they box their army. This can be slightly mitigated by ordering the Changeling to follow the unit it is disguised as within an army, so that it won't be noticed when the army is moved.
Disguised Changelings can only be killed by manually forcing units to attack it (right-clicking on it will issue an Attack command) or waiting for its timer to elapse. Note that the Changeling's Disguise ability is only triggered within a range of 12 and only if vision is provided for it, as the Changeling itself features a sight range of just 8. Units that are "hidden" to the Changeling (e.g. unspotted units on a cliff or cloaked units) will easily dispatch of it in its native form. Also, note that Sieged Tanks can attack up to a distance of 13 and thus can automatically kill an unmorphed Changeling before it can get in range to disguise itself.
Some properties of the Changeling change after it disguises itself. Changelings disguised as Zerglings will move at the speed of a slow Zergling and get a speed multiplier from Creep, though they do not benefit from any player's Metabolic Boost upgrade even if they appear to have wings. Changelings disguised as Marines or Zealots have the sight range of the copied unit, increasing their sight to 9. Finally, Changelings disguised as Zealots will appear to lose their shields to an EMP Round, but will begin regaining them immediately at a rate of 0.5 points per second. This is in contrast to real Protoss units, which will not begin regaining their shields for 10 seconds and will regain them at 2 points per second.
Disguise is a passive ability which is triggered automatically.
The Changeling is most often used to keep track of the opponent's main army, and is usually ordered to follow one of the enemy units (preferably of the same kind). You then get constant updates on the composition and position of their army, so that making unit counters, flanking and harassment are all much easier. These are very effective scouts against an uninformed opponent.
It is possible to create a pseudo Force Field using a group of Changelings to block off a ramp. By using multiple Overseers, it is possible to spawn a large mass of Changelings. Putting these on a ramp on old will require the enemy to manually target all the Changelings. 
Changelings work very well against Protoss since Zealots are normally part of any army and will be hard to spot, allowing for a lot of scouting.
- Use the Changeling against Terran to constantly keep an eye on the unit composition/size of their army. This is very important because it allows you to decide when to Drone or use the Larvae to make the necessary units to counter their army. It also allows you to not waste Larvae on either unnecessary or ineffective units. Zerg will be much better off knowing when that inevitable Terran push is getting ready to move out.
- Use Overseers to drop Changelings on and around Siege Tanks to cause them to shoot themselves and other friendly units with their splash damage. This is a good alternative to using Infestors with Infested Terran. 
Changelings can be more effective against a Zerg player because the Zerglings they morph into are easier to hide and harder to target.
- Changelings are not affected by Zerg carapace upgrades.
- While most attacks will kill unmorphed Changelings in one hit simply by virtue of having the least HP of any unit in the game, disguised Changelings will die in one hit regardless of how little damage is done.
- Changeling's sounds (video and sound) - YouTube video.
- Changeling diguised as a Marine's sounds (video and sound) - YouTube video.
- Changeling diguised as a Zealot's sounds (video and sound) - YouTube video.