Crassula ovata - Jade plant.
Portulacaria afra - Spekboom, also called dwarf or miniature jade plant
Like many succulents, these do not have "true wood" with growth rings and such like an Elm or Oak. They get woody, like some herbs get woody, but it is not "true wood". For this reason, some do not consider them bonsai. For Crassula, the leaves generally do not reduce, so there is always an issue of the proportions not being quite as some would like it. Portulacaria afra has smaller leaves, and can really be trained into a convincing image of a tree. I saw a Portulacaria at a MABA convention, and it "blended in" with the other trees on display. It was near 3 feet tall and nearly as wide, planted on a rock, and it looked like a "proper bonsai", very woody looking trunk.
These days most bonsai clubs do accept Crassula as bonsai if it is well enough developed to look somewhat tree like.
My favorite succulents for bonsai are Bursera, Operculicarya decaryi, and Boswellia species. They are all fat trunk pachycauls, meaning that they have a spongy water retaining layer between the outer bark and the cambium. These are true woody shrubs. They have growth habits that remind you of baobabs, but have smaller leaves and pleasant smelling resins.