Tyres; what difference does a 'Z' make?

[T40ORA]

New boots time and I'm going for Goodyear Eagle F1 GS-D3's at 235/40 R18 91Y.

But the Excellence tyres are 235/40 ZR18 95Y.

Can anyone explain to me the difference between the R and ZR ratings and the difference between 91Y and 95Y? In words of no more than two syllables!

[Maxx-Waxx]

ZR rating for 150 mph,Tyres marked with a "W" or "Y" are designed to go faster than 150 MPH but no faster then 169 & 186 MPH respectively.

Not sure about R, i thought R rated were the lowest speed rating, or it could mean radial

N

[T40ORA]

Spec for both tyres says 186 mph!

[David Godfrey]

wiki has lots of info
http://en.wikipedia.org/wiki/Tire_code