Slavery

A slave was an individual forced to perform tasks. It differed from a free servant, who could quit.

Other species enslaving humans
Humanity had a long and largely unhappy relationship with slavery. Sometimes other species enslaved humans, but there were many examples of one group of humans enslaving another, or of humans enslaving other species.

Daleks forced many people into labour in Bedfordshire, (DW: The Dalek Invasion of Earth) as well as on Skaro itself. (DW: Destiny of the Daleks) They've also occasionally enslaved humans through physiologic manipulation, such as when they have created robomen or "humanoids". (DW: The Dalek Invasion of Earth, DAN: The Humanoids) Relatedly, the Cult of Skaro once attempted to actually blend Dalek and human DNA so as to create a more reliable class of slave labour. (DW: Evolution of the Daleks)

At least one Time Lord — the Master — was able to enslave humans on a massive scale. Though fear alone, he got virtually the entire population of 21st century Earth to help him create what he termed "the New Time Lord Empire", a project that involved massive building works all around the planet. (DW: Last of the Time Lords)

Humans enslaving humans
At different points in their history, some humans enslaved other humans. In 2011, Luke Smith summed up humanity's history of slavery to his sister: "Humans used other humans as slaves for centuries. Every culture, the world over. Nobody ever challenged them until a few hundred years ago. And in some places it still goes on."

- Luke Smith

Humans enslaving other species
In the 21st century, there was a black market in alien slaves on Earth. John Harrison purchased a group of Skullion slaves on this market. (SJA: The Man Who Never Was)

As Rose Tyler observed, humans had developed an industry of enslaving the Ood by the 42nd century, one that was eventually brought down by the Tenth Doctor and Donna Noble. (DW: The Impossible Planet, Planet of the Ood)

Slavery in other species
Many species, such as the Ood (DW: Planet of the Ood) and Mutos, were forced into slave labour. (DW: Genesis of the Daleks)

The Huducts enslaved the Anubians by using mind-controlling torcs. After the Anubians were liberated by K9 Mark I, the Anubians turned on their former masters and enslaved them in the same way, even though K9 intended for them to reconcile in peace. The Anubians later developed space travel and enslaved other species. (K9TV: Curse of Anubis)

The Sycorax practised slavery. During their invasion of Earth on Christmas Day, 2006, they planned to sell half of humanity into slavery. (DW: The Christmas Invasion)

Some races, such as the Daleks, altered individuals into becoming slaves. Example of such are the Robomen, (DW: The Dalek Invasion of Earth) Cybershades (DW: The Next Doctor) and new humans. (DW: New Earth)