To decorate something is to add ornamentation to it or to embellish it in some way. To bedeck something is to go all out with your ornamental efforts, adorning that object (be it a tree, a room, or one’s outfit) to the point of gaudiness. Bedeck is not a very common term; the shorter verb deck, as in “We were all decked out for the party,” is more familiar. But when bedeck does make an appearance, it's likely for a special occasion. This glitzy verb might describe a person who is drenched in jewels, an architectural feature festooned with flowers, or a tree absolutely covered with ornaments.