Roller skis come in all shapes and sizes! Like snow skis
there are all sorts of skis available, depending on what you want to use them
for.
Classic roller skis
Tend to be longer in the shaft than the others. will always have a
ratchet on one of the wheels, usually the front, to give the "grip"
required in the classical technique. The wheels are often made of a
fairly soft rubber and are not therefore renowned for lasting long.
Skating roller skis
Tend to have a shorter shaft. The wheels don't have ratchets.
Combi roller skis
At the simplest this involves the shorter shaft of a skating roller ski,
with a ratchet on one of the wheels. More sophisticated variants come
with a detachable outrigger wheel at the front, to give the extra length.
Racing roller skis
Shaft size is the same as for skating skis (it is very rare to find a
racing ski with a ratchet). But the wheels are much narrower - about a
cm rather than 5cm, and with a diameter of about 10cm rather than 5cm.
Much much more difficult to balance on, and with the option of different
hardness to contribute to more speed.
Off road roller skis
Have pneumatic tyres, and are much bigger all round - wheels are about 15cm
diameter and 5 cm wide. only really cope with hard grassy
surfaces.
Tend to be longer in the shaft than the others. will always have a
ratchet on one of the wheels, usually the front, to give the "grip"
required in the classical technique. The wheels are often made of a
fairly soft rubber and are not therefore renowned for lasting long.
Tend to have a shorter shaft. The wheels don't have ratchets. 
At the simplest this involves the shorter shaft of a skating roller ski,
with a ratchet on one of the wheels. More sophisticated variants come
with a detachable outrigger wheel at the front, to give the extra length.
Shaft size is the same as for skating skis (it is very rare to find a
racing ski with a ratchet). But the wheels are much narrower - about a
cm rather than 5cm, and with a diameter of about 10cm rather than 5cm.
Much much more difficult to balance on, and with the option of different
hardness to contribute to more speed.