What do the numbers mean?

Section width:

The width of the tyre in millimetres.

The aspect ratio (Profile): 

The ratio of the tyre's width to height, as a percentage.

What do I need to know?
A low profile tyre (short sidewall vs width) gives a firmer ride with better handling but tends to be noisier and more expensive than tyres with a higher aspect ratio.

Rim diameter:

The diameter of the rim which the tyre is to be fitted to, in inches.

Load rating:

Indicates the maximum load that each tyre can support.

Speed rating:

The speed rating indicates the maximum speed at which the tyre can carry a load corresponding to its load rating.