Cellulite is part of the skin, normal fat is under the skin
Cellulite is superficial fatty tissue, part of the skin itself and compartmentalised in a collagen sheath. Think of tortellini where the filling represents the fat and the pasta around it is represents the collagen sheath. And then imagine those tortellinis being firmly attached to each other by short spaghetti pieces (collagen fibres). This gives you an idea...
Because they are tethered to each other and to the skin surface, when those fat parcels enlarge they cause the surface of the skin to become lumpy (cellulite appearance).
On the other hand, what people call "fat" is deeper fatty tissue found below the skin, just underneath cellulite. Deep fat is not contained in any sheath and does not have much connective tissue fibres in it, so when it enlarges in does not cause any "bumps" on the skin surface.
The main differences are as follows:
- "Normal" fat can be removed by liposuction. Cellulite, being part of the skin itself, cannot be removed by liposuction (despite what most women are led to believe) and needs special, non-surgical treatment (a multi-ingredient, concentrated, good cellulite cream can also help)
- Normal fat is found everywhere on the body, while cellulite tends to be found on the thigh / butt area (and sometimes on the arms or even stomach)
- Normal fat is found on both sexes - cellulite is usually found in women (due to the effect of estrogen).
In summary, cellulite is a very fibrous type of fat (not "toxins" or just water retention, as you people believe) that is incorporated into the lower layer of the skin itself, hence the mattress-like appearance.