The NBA Draft is the way that National Basketball Association
NBA teams get to select new players in a way that is somewhat fair. The draft is proceeded by a lottery, where the teams with the lowest records are assigned a place in the draft. The rules for how this is done exactly change occasionally. But basically, the teams with the worst records get to pick first, a fairly simple negative feedback loop. Currently, the NBA lottery goes two rounds, meaning sixty players will be picked each year. These players can be picked from a variety of sources: the NBA's minor league, foreign professional leagues, foreign amateur leagues, and players just out of high school. But for the most part, players in the NBA draft will be players of college basketball, specifically of programs in the National Collegiate Athletic Association.
The inputs and the outputs of the NBA draft are pretty simple. There are around 300 NCAA Division I basketball programs in the United States, which means that there are 1500 starters at any given time in these programs. If we look at one-fourth of them "graduating" every year, that gives us around 400 basketball players who might be interested in the NBA draft. That is over 6 times as many players as can go into the draft, but most of these basketball programs are not that successful. Every year, the NCAA holds a tournament colloquially known as "March Madness" to determine the championship team. There are 64 teams in the tournament, which fits nicely with the numbers of the NBA draft. On average, we could look at the NBA draft as selecting one player from each of the teams that goes into the NCAA tournament. Of course, this is not the case: as mentioned, players come to the NBA from other sources, and some players whose teams don't get into the tournament are still drafted. The most successful teams might have multiple players drafted. But a good way to look at the NBA draft is that it will be selecting one or two players from the 10 or 20% of the best teams in college basketball, a year. Or put another way, around 4% of players who start in a NCAA game will be drafted, or a little bit less than 2% of all NCAA basketball players will be drafted.
This is some pretty stringent competition.
As for the outputs of the draft: every team in the NBA has 12 players on their active list, and 15 counting reserves for injured players. There are currently 30 teams in the NBA, meaning that there are either 360 players or 450 players active in the NBA at any given time, depending on how you count. Since the draft adds 60 players to this pool every year, it thus follows that the NBA has an average active lifespan of players of either 6 or 7 years. As with many forms of mortality, this is skewed against the younger players: once a player establishes himself in the NBA, a career of 10 or 12 years is fairly normal. Most of the attrition comes from players who are drafted and never adjust to the NBA, due to injuries or difficulty in transitioning to professional basketball.
In other words, the NBA draft is a way to select the best basketball players in both the United States and the world, and out of the pool of available players, which probably numbers between 3-5000 at any time, only 60 will be selected. And because established players in the NBA can have rather long lifespans in the sport, many, if not most of those 60 players will have cursory careers, and some will never play at all. It is a rather viscous sieve, but very rewarding for the two dozen or so players a year who do manage to make it as core NBA players.