Since you are asking what the difference is I thought I'd point out one of the most important differences between the two kinds of spheres in relation to modelling.
Comparison using many subdivisions
UV spheres are best used in situations where you require a very smooth, symmetrical surface (the more subdivisions the better). However, they do relatively poorly when realistic topology is your key goal. UV spheres are not the best for organic natural shapes, because of their symmetry and the use of quads.
ICO spheres are best used for objects such as:
- Geodesic Domes
- Planets with realistic terrain
- Rough surfaces, such as golf balls.
If symmetry is important, use UV spheres because ICO spheres are asymmetrical.
A displacement modifier on the ICO sphere yields more satisfying results. With a higher number of subdivisions the difference between ICO and UV sphere won't be as noticeable. Generally, though, the ICO sphere looks more realistic and much smoother.
Displace modifier settings