How was slaves used within the dar al Islam?

Dar al Islam - Domain of Islam, land currently under Islamic rule Dar al Harb - Domain of War, land currently not under Islamic rule

The al- is an Arabic prefix which means the. It is used to define the noun, so literally, al-Islam means the submission. It is usually omitted when telling it in English.

because people were too lazy to do the work themselves!

They were used to do un-paid labor works (such as mining, farming; first sector jobs). Some of them were also involved in second sector jobs(Manufacturing). The works of the slaves were ultimately...

Slave owners.