A slack variable is added to a less-than-or-equal-to (≤) constraint to convert it into a standard equation. 

Because a less-than-or-equal-to constraint ($\le$) indicates that the left-hand side is less than or equal to the available capacity on the right-hand side, adding a non-negative slack variable absorbs this unallocated difference or "slack" to make both sides exactly equal.

You add a slack variable to a less-than-or-equal-to constraint to turn it into a standard equality constraint.

and what type of variable is subtracted from a greater-than-or-equal-to constraint? 

A slack variable is added to a less-than-or-equal-to (≤) constraint to convert it into a standard equation. It represents the unused amount of a resource.

A slack variable is added to a less-than-or-equal-to constraint to convert it into a standard equation. It represents the unused amount of a limited resource and is added to the left side of the constraint so that the inequality becomes an equality.

Slack variable. It is added to a ≤ constraint to turn it into an equation.

A slack variable is introduced to a less-than-or-equal-to connstraint to change it into an equation. It indicates the unused portion of a limited resource and is added to the left side so that the inequality becomes an equality.