In reply to PanzerHanzler:
Yep, I use one on longer runs so I can stay hydrated.
Originally they were all small rucksacks with a sleeve for the bladder.
In the last 2 or 3 years there the trend is more "vests" rather than "rucksacks" - a lot closer fitting, sitting quite high on the body, no waist strap. Most will still take a bladder, but in the last year or so the trend has been more towards front mounted bottles, or soft flasks.
Problem with bladders is you don't know how much water you have left, and difficult to fill up on the run.
For shorter long runs I still prefer the bladder rather than having the water sloshing about in bottles on my chest ( not tried the soft flasks which are supposed to prevent the sloshing as they collapse as the water levels decrease ). For really long runs where you need to fill up then would go with the bottles for ease of re-filling from streams.