Remember:
All cacti are succulents,
but not all succulents are cacti!
This is because cacti develop their spines from a very special
organ that no other plant has - it's called an areole. Succulents
with spines grow them in a completely different manner.
The
succulent Sempervivum tectorum is also called House Leek. In the
olden times, the roofs of houses were often covered with turf
or sod. A turf roof would be planted with house leek to keep evil
spirits away, these days a house leek is often planted near the
front and back door of a home to do the same job. It was also
said you would never lose your home if house leek guarded it,
and house leek is almost impossible to transplant, once settled.
- Aren't you glad you don't have to go outside and "water your
roof"!
The
young tender paddles of the Opuntia cactus are edible and eaten
throughout Mexico in a variety of ways. The fruits of this plant
are also edible and sometimes called Cactus Apples. Their flesh
is dark red and very sweet and can be eaten just as it is - it
also makes a terrific ruby colored jelly! 
The
same Opuntias were taken to Australia many years ago and planted
as a food source for cattle. Though it sounded like a good idea,
it became an ecological nightmare. The cactus liked it's new home
so well that it proceeded to take over the outback and it's native
plants. The Aussies have been tearing it out and burning it down
for over 30 years now and they still can't get rid of it - Cracky
Mate!
Plants
with windows? There are a number of succulents, Haworthias, Baby
Toes, Peperomia, and Lithops to name a few that actually have
clear windows on their leaf tips. This is because they come from
extremely harsh climates where they need to be able to collect
as much light as possible in the wintertime to survive. This allows
all available light to penetrate deep inside the body of the plant.
1
Hippopotamus, 2 or more Hippopotami
1 Octopus, 2 or more Octopi
1 Cactus, 2 or more Cacti
Spines
on cacti are not only for protection from predators but also protection
from the sun. Even though the spines may be thin, as the sun moves
all day long each spine is always shading a different part of
the plant. This amounts to fairly even shading over the course
of the day. Succulents, on the other hand, usually rely on spines
and thorns for defense against critters, but will use things like
dense wooly or fuzzy coatings on their stems and leaves for sun
protection; Kalanchoes are famous for this. 
Botanical
names are the only way to truly identify a plant, but common names
can be entertaining:
Beaver
Tail - Opuntia basilaris
Devil's Tongue - Ferocactus latispinus
Mule Crippler - Echinocactus horizonthalonius
Golden Torch - Trichocereus spachianus
Mother in Law's Tongue - Gasteria Family
Mother and 10,000 Babies - Several Kalanchoe species
Pregnant Onion - Ornithogalum longibracteatum
Some
years back, a hunter was shooting quail in the deserts of Arizona,
home of the Giant Saguaro cactus, which is protected by law
in the wild. The hunter decided to have some target practice and
took several shots at one of the giant cacti with his shotgun.
When he was done, he turned to leave and the cactus fell directly
on top of him - he died. Mother Nature does get even!
The
Agave is one of the most versatile succulents. In ancient times
in the Americas it's thorns were used for needles, the fibers
for thread or woven into fabric or ropes, and the flesh baked
and eaten or turned into a beverage - we know that one today as
Tequila! Also the large Maguey worms that bore into
these plants (the larvae of the Giant Skipper Butterfly)
are still very much enjoyed today deep fried and served
straight up with a squirt of lime juice, or as a filling for tacos!
Cacti
are native only to the Americas from as far north as northern Canada all the way south to the tip of Patagonia, South America's very southernmost point. If you see cacti in any other part of the world, somebody brought it there.
Succulents
on the other hand have always been found on every continent of
the planet. Some even took a course of parallel evolution. In
Africa for example, Euphorbias (though succulents) fill a similar
place in nature as cacti in the Americas. Euphorbias however are
definitely NOT EDIBLE!
