My mother worked at a Pioneer Camp for many years, so for the most part I'll provide the information she remembers and will add something from other sources, 1 and 2.

You are a Pioneer, which means you're from 7 to 15 years old. To become a Pioneer, you should have decent grades and don't violate the school rules. If you were not accepted, it means you did something outrageous.

Almost all Pioneer camps were managed by some organizations. If your parents, like my mother, worked in a big construction company, you'd most likely go to the Pioneer Camp that belonged to that company. Normally, a Pioneer camp was a big territory in the forest, close to a river or a lake, so you should expect lots of swimming, sports, games, competitions, and other events. Normally, you'd stay for 21 days, and the biggest event in these 21 days will be "Zarnitsa" - a big quest that combined sports, basic military training, I think, the scouts in the US have similar experience. By the way, the Pioneer movement was organized by the members of the Scout movement in the early 1920s, so many things would be similar.

You wouldn't live in tents, the camps were more like the summer resorts, so you'll have a cafeteria, library, dancing hall, etc. Depending on your Leader and the admisitration of the camp, your whole experience would be more or less organized and official, but more often it was definitely not boring.

If you are extremely lucky or your parents are important, you'll go to a Pioneer Camp to the Black sea and all your friends will be jealous when you get back. From all Pioneer Camps, "Artek" was the most famous and hard to get into.

Atteding a Pioneer Camp wouldn't somehow affect your future. Unlike scouts, Pioneers didn't have badges, medals, etc., because the group was more important than the individual.

Normally, you'd be eager to attend, but if you got to a Camp where everything is very official and strict, your overall experience won't seem as exciting. You had some alternatives to a Pioneer Camp: -going to the village to your grandparents, where you could play all by yourself and make new friends or working with your grandparents. -going for a vacation together with your parents, either to the South of the country or to a touristic place.

There were other options, like going to the South or visit Moscow or St. Petersburg with your classmates and 2 or 3 adult Leaders. I'll try to get some more information and add it to the comments later.