Here you can find out about the first in the Pokemon Game Series to hit the UK & US! Pokemon Red & Blue Versions

Differences
In Pokemon Red you can only catch the following Pokemon in this Version -
Oddish > Gloom > Vileplume
Growlithe > Arcanine
Mankey > Primeape
Ekans > Arbok
Scyther
Electabuzz

In Pokemon Blue you can only catch the following Pokemon in this Version -
Sandshrew > Sandslash
Meowth > Persian
Bellsprout > Weepinbell > Victreebel
Vulpix > Ninetales
Magmar
Pinsir

The following Pokemon can be only fought in trainer battles -
Meowth > Persian (R)
Sandshrew > Sandslash (R)
Vulpix > Ninetales (R)
Bellsprout > Weepinbell > Victreebel (R)
Growlithe > Arcanine (B)
Oddish > Gloom > Vileplume (B)
Mankey > Primeape (B)
Ekans > Arbok (B)

The Pokemon with a (R) next to the, can only be seen in Pokemon Red Version. The Pokemon with a (B) next to them can only be seen in Pokemon Blue Version.