It's probably important here to distinguish between Selenium, Selenium Webdriver, and Watir Webdriver.
Selenium by itself is more of a passive testing framework. For years, Watir enabled testing by "driving" (automating) the browser, so that you could actually interact with the page (click buttons, select from dropdowns, etc.).
In the meantime, Selenium was developing their Selenium Webdriver, which now does what Watir once did internally.
The decision (a questionable one, IMO) was made by the Watir lead developer to switch out the core webdriver to Watir webdriver. This allows some more flexibility than the old Watir (like the ability to go headless with Ghost), but at the same time it lost many of its important features (like the ability to examine the HTML response directly, which Selenium Webdriver team has long refused to do for misguided "philosophical" reasons).
So, after that background: while Selenium testing framework has been around longer, Watir was the original "webdriver" which allowed your code to automate pages by interacting with the page as though you were sitting at the keyboard. Selenium has caught up a lot in that regard with its Selenium Webdriver (which again is separate from the Selenium IDE), and Watir now (unfortunately) utilizes that webdriver within "Watir Webdriver", which is essentially a Ruby wrapper.
There used to be more difference than just "Ruby or something else". Incorporating Selenium Webdriver relieved some pressure on development of the "driver core" for Watir, at the expense of some the former superior functionality of Watir.
Short answer: not much difference in capability these days.