Heartless

The Heartless  are physical, living manifestations of the darkness within people's hearts.

They manifest in two forms; "Pureblood" and the artificially created "Emblem" Heartless.

They behave entirely devoid of emotion, hence the name "Heartless".

They are the main antagonist of the Kingdom Hearts series as they made more appearance then any enemy types.

About
Heartless, as they seek and consume hearts, desire to return to the greatest heart, the heart of all worlds, Kingdom Hearts, that resides deep within the Realm of Darkness.

And because of this, Heartless seek immense darkness, enough to completely consume the Realm of Light.

The entryway into this place of great power is the Door to Darkness, the door that separates the realm of light and the realm of darkness.

However, with the efforts of Sora and the King, the door is closed, which causes the worlds lost to the darkness to be restored, and preventing a massive army of Heartless from pouring into the realm of light.

The forces of the Heartless were extremely weakened after the door to Kingdom Hearts was sealed off, though there were still many left.

At the end of Kingdom Hearts II, it is unknown what exactly happened to the Heartless and if they still pose a threat.

It should be noted though that Yen Sid has mentioned that the only true way to destroy all Heartless is to have all people have light filled in their hearts and no darkness, leaving nothing to create a Heartless or something for them to thrive for.

Different Types
The Pureblood Heartless are natural Heartless, born when people's hearts are normally consumed by darkness. They are ink-black in color, with yellow beady eyes. They are more common in places that are close to or saturated in darkness. Upon their destruction, they simply disappear in puffs of darkness.

The Emblem Heartless are Heartless created in Ansem's Lab. They are usually more powerful than normal Heartless and they all bear the Heartless Emblem. They were originally created by Xehanort and possibly the other apprentices.

Status
Active, at most.