'Cedar' is a generic term that is used to refer to several types of trees, most of which are not true cedars (Cedrus species) at all. On the west coast, 'cedar' most likely refers to Western red cedar, Thuja plicata. In other parts of the country, 'cedar' usually refers to some sort of native juniper.

Both contain a lot of essential oils that repel insects, primarily by smell. But neither type repel ALL insects. Mollusks - like snails and slugs - are not typically affected, although their presence may be reduced to some extent just because they are not inclined to creep over the rough surface. However, I would not rely on ANY type of cedar mulch to deter slugs/snails -- you need to bait for these types of garden pests.

Insects that will be deterred are some types of ants, cockroaches, termites, fleas, moths, some varieties of flies. Cedar wood chips (either type) are often used as pet bedding because of this property. Also some desirable garden inhabitants, like butterflies, will avoid cedar. But other insect pests will find either type of cedar a welcome habitat, as they will burrow and live in the moist, dark recesses of the thick mulch layer and feed on the decomposing wood. These includes earwigs, vine weevils, pill or sowbugs (roly polys) and various beetles.

Western red cedar is a valuable timber tree with very durable, long-lasting wood. Western red cedar mulch is a by-product of this industry but still tends to be pricey. Eastern red cedar is a weed tree and mulches made of this type of wood tend to be rather inexpensive.