Mule, a French word, is a style of shoe that is backless and closed-toed. Mules can be any heel height - from flat to high. It differs from sandals because they are backless without straps or thongs passing around the ankle or heel.
A slide is a shoe that is backless and open-toed, essentially an open-toed mule.

The sexy version of slides and mules is usually open-toed, with heels.

Example: character wearing sexy mules