also i'm assuming in your original post you meant 190/55 vs 190/50 size?
as others said, the 180 is more fitting for a 5.5" rim and 600/750 sized bikes (street). the 190 or 200 is more for the 6" rims of a liter bike.
the 55 and 50 are the aspect ratio- basically how tall the sidewall is compared to the width.
the original sizes were 180/55-17 and 190/50-17 but with more power came the need/desire for a bit more bite when leaned over, so they have moved to a 190/55-17 size that will give you a bit more traction leaned over. But if it's for the street or light track day usage, 99% of the people will never tell the difference.
and as someone else pointed out, it's all only relavant to a specific manufacturer. the tolerances can be +/- 10% from the official "spec" of the size from ETRTO so there's a lot of variation from brand to brand.