Hi Bro,

you have to break these numbers in to portions to understand their meaning. So let me help you like this. (Info provided is the most common one's, if you want more details you can research further with our sifu Sensei Google.)

the first set of three digits (185 or 195) refers to the width of the tyre. (Commonly people prefer 195 rather than 185, below pro's and cons are based on 195 size perspective)
Pro - More rubber on the surface = better pickup on straights, stable drive during turns, better braking capability.
Cons - Slightly more expensive, requires more torque to move.

the second set of two digits (65/55) is the height of the tire between the last point of the tyre toucing the tarmac till rim. (Commonly people prefer 55 rather than 65, below pro's and cons are based on 55 size perspective)
Pro - a low profile look, faster torque response (Pickup), faster braking capability, cheaper since most people buy this sizes.
Cons - Thread wears faster, car body tends to hit KNNCCB sharp humps, sucks more fuel

the 3rd set of two digits (15) refers to your car rim size.

Personally for me, I'd prefer the 195/50/15 setup due to the pro's I get when it comes to safety.
p/s: Money can earn, life you only have 1.

I know some might not agree with my feedback, again I highlight this is my personal preference.