I have heard pilots talk about flaps and slats, seemingly interchangeably. Is there a difference between a flap and a slat?
Flaps are at the back of the wing, slats are at the front.
If you want it more technical: They both help to generate more lift. See these diagrams with angle of attack and lift coefficient:
As the lift coefficient is inversely proportional to the minimum airspeed, a higher CL will allow a lower Vmin.
Slats and flaps both change the shape of the wing when they are extended and allow the wing to generate more lift so that the airplane can fly slower.
Operationally they are both retracted and flush against the wing except for takeoff and landing at which time they are extended.
Pilots tend to refer to them together because they are used for the same purpose and used at the same time. In fact, they are normally even moved by using the same control (typically the slats comes out when selecting first detent, and then the flaps come out progressively further at each detent):
Slats are on the front of the wing (on the left in this picture) and the flaps on on the back of the wing (on the right). As you can see, they both tend to make the wing bigger and more curved when they are extended: