Will the 26" tyre fit my 25" rim bike? I have this 2002 Esperia which, and I need to change the tires cause I am getting back to it. The tire dimensions appear to be 25 x 1.90, 50-559. I read a bit about these new and old dimensions specifications, but I cannot find 25" (or col, I am not sure whether it's inch or col) in the online stores, but I can find 24" and 26".

Any advice? Will all (or none?) 26" fit my rim or what? 

Additional question - I plan to ride on roads. Do I get slick tires?
The significant numbers are the "50-559" which translate to 26" x 1.90" – my best guess is that what you're seeing is a molding problem that causes the '6' to look like a '5'. The 50-559 is the ISO size of the tire. The first number is the width (50 mm) and the second is the "bead seat diameter" (559 mm).

Beware of 26" tires – there are two sizes that are called 26" and they are not compatible. The easiest ways to tell them apart are by the width where you will see either a decimal width, say 1.50", or a fractional width, say 1 1/2". The other method is to compare the ISO size which would be something like 37-559 for a 26 x 1.5 inch tire or 37-590 for a 26 x 1 1/2.

For riding on roads slick tires are nice, they give you the largest contact area for a given tire size. You need to balance that against durability and availability. Tires like the Schwalbe Marathon are built for durability, but aren't really slicks (at least not in the sense of a tire like the Kojak). Do look for a tire with a "slick" strip down the middle which is where you'll be riding most of the time.