Omnitrix

The Omnitrix is the mysterious alien device that allows the user to transform into various alien forms by combining the DNA of the selected alien with the user's own DNA, effectively turning them into the alien species the user has selected. The alien form is selected by unlocking the control dial, turning the control dial to the desired alien spieces which is displayed on the Omnitrix's display, then pressing the control dial back into place completing the transformation.
The Omnitrix was originally created by Azmuth (a Galvan like Greymatter) to allow the people of the universe a way to literally "walk in someone else's shoes". However, many beings saw the Omnitrix's abilities as something that could be used as a powerful weapon--most prominently the villain Vilgax. Fearing that people might misuse the Omnitrix for this very reason, Azmuth put several safeguards in place to ensure that whoever used the Omnitrix would not have full access to its powers. Several of these safeguards include transforming into an alien different from the one that is selected on the control dial, the ten minute auto time-out, and the restriction of accessible aliens. Even the Omnitrix's self destruct was a safeguard set in place by Azmuth. However, these safeguards are disabled once the Master Control for the Omnitrix is unlocked, which is apparently done by a random combination of selections on the control dial. Once the Master Control is unlocked, the user can stay in their alien form for any length of time and is also able to switch between alien forms at will just by thinking about it.
In addition to the aliens that were pre-programmed into the Omnitrix's DNA sequencer by Azmuth, the Omnitrix also possess the ability to absorb DNA samples from other alien species by simply coming into contact with them, effectively downloading a DNA sample and allowing access to that form.
To date, Ben has unlocked the Master Control only once by complete accident. This occurred during the Season 2 finale episode while messing around with the control dial, though to make sure Vilgax wouldn't have access to the Master Control, Ben zeroed out the Master Control, putting back into place all of Azmuth's safeguards.
In the episodes "Ben 10,000" and "Ken 10", Ben's future self has managed to not only unlock the Master Control, but has access to 10,000 different alien species. Also, his Omnitrix's look has been altered somewhat, though it appears to still function the same.It also seems to have grown and transformed until it reached the top of his forearm. His alien forms seem to have transformed as Ben got older.