I have been in the world of aviation for a while, always able to recognize differences between aircraft, but I still have a doubt.
Some planes, in the specific case of Boeing and Airbus, have the series of, lets say 727-200, and it also had 727-100, but when I see some of the pictures here on the gallery, they appear as 727-264, or other number in the series. When I see the pictures the aircraft type appears as 737-4Q8 for example, not as B737-400.
Airbus is also like that, an Airbus 330-300 would appear as A330-323, or something alike.
I wonder how to interpret the number of the series, if those numbers mean minor changes or something like that, if anybody knows what that means, or if anybody knows a link with information on that it would be helpful.
I can tell between a 747-200 and a 747-300, but not between a 747-432 and a 747-474  

thanks
apg