Russia is using several arenas and stadiums to host matches at the 2018 World Cup.
In North America smaller venues are called arenas. Ice hockey, and basketball games are played in arenas where there's usually no more than 20000 seats, while baseball and American football are played in stadiums with no roof an much bigger seating capacity.
However, bigger venues can also be called arenas. I've also noticed that the arenas of stadium size are often in circular or oval shape, examples are Ekaterinburg and Kazan arena from the link in your question.
A trend is noted (for example on Wikipedia) that arena tends to describe a smaller space with frequent non-sporting usage, much more likely to be roofed, while stadium describes larger spaces with little or no non-sporting usage, often roofless.
However, there are venues that look like most "arenas" which are named "stadium" and vice versa. The only real difference appears to be the name itself, and that is more of a marketing keyword as to the type of events held at the venue.