Well, as far as ornamental plants are concerned, a hybrid is usually a cultivar; but a cultivar doesn't have to be a hybrid!
Now, is that perfectly clear?
OK, the word "cultivar" is short for "cultivated variety" and a cultivar is a selected form of a plant that has some desirable characteristic for cultivation. It is often given an attractive name by an enterprising horticulturist with the hope of commercial success. The desirable characteristic might be flower colour, growth habit, foliage characteristic, disease resistance or anything else. The cultivar may be an unusual form of a pure species that was discovered in the wild, a form that arose in a batch of seedlings in a nursery or a hybrid...so a cultivar doesn't have to be a hybrid.
A hybrid is a genetic "cross" between two different species (usually within the same genus) or between a species and other hybrids. Hybrids can occur naturally or they can be produced by human intervention. The aim of deliberate hybridization is to produce plants with desirable characteristics. If the hybridization gives successful results, the plant is usually given a cultivar name and released commercially....so, a successful hybrid is also a cultivar.
As examples, Grevillea banksii is a pure species, Grevillea banksii "Ruby Red" is a cultivar selected for its prostrate habit and Grevillea "Superb" is a hybrid cultivar between G.bipinnatifida and G.banksii.
|Left: Grevillea banksii. Right: Grevillea 'Superb'. Photos: Brian Walters|