It is half-competitive and half-fun event. There is lower age limit on 7km and
21.1km distances, but other than that (and barring medical conditions) anyone can join and try to run chosen distance. It also happens on 7th of January regardless of the weather - this year it was -8C, but the coldest one was run in 2001 in -39C, but even then 13 runners took on a full distance.
I am not a runner, but I like watching the start of the run on TV, because there are always people in costumes that a worth watching. This year I originally wanted to go there in person... but I got buried in work. So once again I had to settle for TV translation.
There were some nice costumes, here is a link to an article with photos, you really should take a look at photos there.