It's quite the range: There are highly regarded professional leagues across Europe with lots of grown men, including Russia's Kontinental Hockey League (KHL) and the top Finnish league, Liiga, which is called the Finnish Elite League here in North America. Teens who play in these leagues are considered prime prospects for the NHL.
While NCAA hockey sends its share of players to the NHL, "junior" hockey is one element of stocking NHL rosters that differs greatly from the NBA, NFL and MLB.
Junior hockey generally includes players ages 16 to 21 who are amateurs and who play in U.S. and Canadian leagues. "Major junior" hockey is the highest level of play in Canada, where this whole junior system started. The major junior leagues in Canada are the aforementioned WHL (operating in western Canada provinces, Washington and Oregon), OHL (Ontario) and QMJHL (Quebec). These three leagues comprise the CHL or Canadian Hockey League. There is also a second tier of juniors in Canada known as Junior A.
In the U.S., the top junior hockey league is the USHL, which has 16 teams located in Midwest states and is sanctioned by USA Hockey, the governing body for amateur play here in the States.
All junior hockey teams and leagues are amateur level, allowing players to be drafted and, in some cases, attend NCAA schools before, if they are good enough, turning pro. Hockey is unique among the four major pro sports leagues in that its junior level is independent of school and that the majority of NHL draftees come from outside the collegiate system. Junior hockey leagues exist strictly to develop hockey players.