What is the difference between a boutonniere and a button hole

There is really no difference between a button hole and boutonniere.

What should or can it look like? What accessories can be added?

A buttonhole or boutonniere is usually a single flower which is pinned to the lapel of the grooms jacket. The boutonniere dates back to Greece and Rome when youngsters picked and added branches of flowers to their togas on their way to celebrations.
Rose artificial wedding button hole
A lot of time has passed since then, and although very sweet, modern brides and grooms are unlikely to pick their wedding flowers on the way to their wedding as the flowers and other details should match each other and share the same theme. A boutonniere or buttonhole should match the colour scheme of the wedding and go together with a bridal bouquet.
The additional elements to decorate a boutonniere could range from ribbons, beads, feathers or pearls, in other words, any accessories, that will help to match the wedding gown and the groom suit.
If a single-stem flower in the groom suit is entwined with an elegant satin ribbon, this ribbon could be repeated on the wedding bouquet, or could be of the same colour as the bridesmaids or the bridal dress. If you use the decorative beads to embellish the buttonhole, they could also be incorporated in the wedding bouquet or the bridal hair accessories.
It is a challenge to pick up the well-matched elements without the professional help of the stylist or the florist. If you do it alone,  browse through the web to see successful examples of boutonnieres decorated.